diff --git a/examples/dodrio/counter/Cargo.toml b/examples/dodrio/counter/Cargo.toml
index c9a2870..5a16a25 100644
--- a/examples/dodrio/counter/Cargo.toml
+++ b/examples/dodrio/counter/Cargo.toml
@@ -2,7 +2,7 @@
 name = "dodrio-counter"
 version = "0.1.0"
 authors = ["Nick Fitzgerald <fitzgen@gmail.com>"]
-edition = "2018"
+edition = "2021"
 
 [lib]
 crate-type = ["cdylib"]
diff --git a/examples/dodrio/todomvc/Cargo.toml b/examples/dodrio/todomvc/Cargo.toml
index eae203a..12536fb 100644
--- a/examples/dodrio/todomvc/Cargo.toml
+++ b/examples/dodrio/todomvc/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 authors = ["Nick Fitzgerald <fitzgen@gmail.com>"]
-edition = "2018"
+edition = "2021"
 name = "dodrio-todomvc"
 version = "0.1.0"
 
diff --git a/examples/iron/Cargo.toml b/examples/iron/Cargo.toml
index 04f80a7..6a8fadb 100644
--- a/examples/iron/Cargo.toml
+++ b/examples/iron/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "typed-html-iron-test"
 version = "0.1.0"
-edition = "2018"
+edition = "2021"
 authors = ["Bodil Stokke <bodil@bodil.org>"]
 
 [dependencies]
diff --git a/examples/stdweb/Cargo.toml b/examples/stdweb/Cargo.toml
index 57a9843..8eb27fa 100644
--- a/examples/stdweb/Cargo.toml
+++ b/examples/stdweb/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "typed-html-stdweb-test"
 version = "0.1.0"
-edition = "2018"
+edition = "2021"
 authors = ["Bodil Stokke <bodil@bodil.org>"]
 
 [dependencies]
diff --git a/macros/Cargo.toml b/macros/Cargo.toml
index 806cb98..5b04056 100644
--- a/macros/Cargo.toml
+++ b/macros/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "typed-html-macros"
 version = "0.2.2"
-edition = "2018"
+edition = "2021"
 authors = ["Bodil Stokke <bodil@bodil.org>"]
 build = "build.rs"
 license = "MPL-2.0+"
diff --git a/macros/src/config.rs b/macros/src/config.rs
index d486414..8a86b9c 100644
--- a/macros/src/config.rs
+++ b/macros/src/config.rs
@@ -21,12 +21,15 @@ pub fn global_attrs(span: Span) -> StringyMap<Ident, TokenStream> {
 
         insert("accesskey", "String");
         insert("autocapitalize", "String");
-        insert("contenteditable", "crate::types::Bool");
+        insert("contenteditable", "crate::types::EnumeratedBool");
         insert("contextmenu", "crate::types::Id");
         insert("dir", "crate::types::TextDirection");
-        insert("draggable", "crate::types::Bool");
+        insert("draggable", "crate::types::EnumeratedBool");
         insert("hidden", "crate::types::Bool");
         insert("is", "String");
+        insert("itemprop", "String");
+        insert("itemscope", "crate::types::Bool");
+        insert("itemtype", "String");
         insert("lang", "crate::types::LanguageTag");
         insert("role", "crate::types::Role");
         insert("style", "String");
@@ -39,20 +42,6 @@ pub fn global_attrs(span: Span) -> StringyMap<Ident, TokenStream> {
 }
 
 pub static SELF_CLOSING: &[&str] = &[
-    "area",
-    "base",
-    "br",
-    "col",
-    "command",
-    "embed",
-    "hr",
-    "img",
-    "input",
-    "keygen",
-    "link",
-    "meta",
-    "param",
-    "source",
-    "track",
-    "wbr",
+    "area", "base", "br", "col", "command", "embed", "hr", "img", "input", "keygen", "link",
+    "meta", "param", "source", "track", "wbr",
 ];
diff --git a/macros/src/declare.rs b/macros/src/declare.rs
index 819b039..d7aaf81 100644
--- a/macros/src/declare.rs
+++ b/macros/src/declare.rs
@@ -15,6 +15,7 @@ pub struct Declare {
     pub attrs: StringyMap<Ident, TokenStream>,
     pub req_children: Vec<Ident>,
     pub opt_children: Option<TokenStream>,
+    pub opt_children_dyn: bool,
     pub traits: Vec<TokenStream>,
 }
 
@@ -24,6 +25,7 @@ impl Declare {
             attrs: global_attrs(name.span()),
             req_children: Vec::new(),
             opt_children: None,
+            opt_children_dyn: false,
             traits: Vec::new(),
             name,
         }
@@ -34,11 +36,7 @@ impl Declare {
     }
 
     fn attr_type_name(&self) -> TokenTree {
-        Ident::new(
-            &format!("Attrs_{}", self.name),
-            self.name.span(),
-        )
-        .into()
+        Ident::new(&format!("Attrs_{}", self.name), self.name.span()).into()
     }
 
     fn attrs(&self) -> impl Iterator<Item = (TokenTree, TokenStream, TokenTree)> + '_ {
@@ -99,7 +97,12 @@ impl Declare {
 
         if let Some(child_constraint) = &self.opt_children {
             let child_constraint = child_constraint.clone();
-            body.extend(quote!(pub children: Vec<Box<#child_constraint<T>>>,));
+            let child_dyn = if self.opt_children_dyn {
+                quote!(dyn)
+            } else {
+                quote!()
+            };
+            body.extend(quote!(pub children: Vec<Box<#child_dyn #child_constraint<T>>>,));
         }
 
         quote!(
@@ -170,7 +173,7 @@ impl Declare {
         for (attr_name, _, attr_str) in self.attrs() {
             push_attrs.extend(quote!(
                 if let Some(ref value) = self.attrs.#attr_name {
-                    attributes.push((#attr_str, value.to_string()));
+                    attributes.push((#attr_str, value.to_string_value()));
                 }
             ));
         }
@@ -219,7 +222,7 @@ impl Declare {
         for (attr_name, _, attr_str) in self.attrs() {
             push_attrs.extend(quote!(
                 if let Some(ref value) = self.attrs.#attr_name {
-                    out.push((#attr_str, value.to_string()));
+                    out.push((#attr_str, value.to_string_value()));
                 }
             ));
         }
@@ -303,26 +306,14 @@ impl Declare {
         }
 
         let print_children = if self.req_children.is_empty() {
-            if self.opt_children.is_some() {
-                if !SELF_CLOSING.contains(&elem_name.to_string().as_str()) {
-                    quote!(
-                        write!(f, ">")?;
-                        #print_opt_children
-                        write!(f, "</{}>", #name)
-                    )
-                } else {
-                    quote!(if self.children.is_empty() {
-                        write!(f, " />")
-                    } else {
-                        write!(f, ">")?;
-                        #print_opt_children
-                        write!(f, "</{}>", #name)
-                    })
-                }
-            } else if !SELF_CLOSING.contains(&elem_name.to_string().as_str()) {
-                quote!(write!(f, "></{}>", #name))
+            if SELF_CLOSING.contains(&elem_name.to_string().as_str()) {
+                quote!(write!(f, ">"))
             } else {
-                quote!(write!(f, "/>"))
+                quote!(
+                    write!(f, ">")?;
+                    #print_opt_children
+                    write!(f, "</{}>", #name)
+                )
             }
         } else {
             quote!(
@@ -337,10 +328,7 @@ impl Declare {
         for (attr_name, _, attr_str) in self.attrs() {
             print_attrs.extend(quote!(
                 if let Some(ref value) = self.attrs.#attr_name {
-                    let value = crate::escape_html_attribute(value.to_string());
-                    if !value.is_empty() {
-                        write!(f, " {}=\"{}\"", #attr_str, value)?;
-                    }
+                    value.write_attribute(#attr_str, f)?;
                 }
             ));
         }
diff --git a/macros/src/grammar.lalrpop b/macros/src/grammar.lalrpop
index 172313e..983c059 100644
--- a/macros/src/grammar.lalrpop
+++ b/macros/src/grammar.lalrpop
@@ -272,8 +272,8 @@ IdentList = "[" <Separated<Ident, ",">> "]";
 
 Groups = "in" <TypePathList>;
 
-Children: (Vec<Ident>, Option<Vec<Token>>) = "with" <req:IdentList?> <opt:TypePath?> => {
-    (req.unwrap_or_else(|| Vec::new()), opt)
+Children: (Vec<Ident>, bool, Option<Vec<Token>>) = "with" <req:IdentList?> <has_dyn:"dyn"?> <opt:TypePath?> => {
+    (req.unwrap_or_else(|| Vec::new()), has_dyn.is_some(), opt)
 };
 
 Declaration: Declare = <name:HtmlIdent> <attrs:Attributes?> <groups:Groups?> <children:Children?> ";" => {
@@ -288,8 +288,9 @@ Declaration: Declare = <name:HtmlIdent> <attrs:Attributes?> <groups:Groups?> <ch
             decl.traits.push(to_stream(group));
         }
     }
-    if let Some((req_children, opt_children)) = children {
+    if let Some((req_children, has_dyn, opt_children)) = children {
         decl.req_children = req_children;
+        decl.opt_children_dyn = has_dyn;
         decl.opt_children = opt_children.map(to_stream);
     }
     decl
@@ -321,6 +322,7 @@ extern {
         "]" => Token::GroupClose(Delimiter::Bracket, _),
         "in" => Token::Keyword(lexer::Keyword::In, _),
         "with" => Token::Keyword(lexer::Keyword::With, _),
+        "dyn" => Token::Keyword(lexer::Keyword::Dyn, _),
         IdentToken => Token::Ident(_),
         LiteralToken => Token::Literal(_),
         ParenGroupToken => Token::Group(Delimiter::Parenthesis, _),
diff --git a/macros/src/html.rs b/macros/src/html.rs
index eefa817..d97b251 100644
--- a/macros/src/html.rs
+++ b/macros/src/html.rs
@@ -379,7 +379,7 @@ impl Element {
             let key_str = TokenTree::from(Literal::string(&key_str));
             builder.extend(quote!(
                 let attr_value = dodrio::bumpalo::format!(
-                    in &#bump, "{}", element.attrs.#key.unwrap());
+                    in &#bump, "{}", typed_html::types::DisplayAttribute::to_string_value(&element.attrs.#key.unwrap()));
                 if !attr_value.is_empty() {
                     attr_list.push(dodrio::builder::attr(#key_str, attr_value.into_bump_str()));
                 }
diff --git a/macros/src/lexer.rs b/macros/src/lexer.rs
index 4a2a412..f27ac18 100644
--- a/macros/src/lexer.rs
+++ b/macros/src/lexer.rs
@@ -82,6 +82,7 @@ impl From<Group> for Token {
 pub enum Keyword {
     In,
     With,
+    Dyn,
 }
 
 pub fn keywordise(tokens: Vec<Token>) -> Vec<Token> {
@@ -94,6 +95,8 @@ pub fn keywordise(tokens: Vec<Token>) -> Vec<Token> {
                     Token::Keyword(Keyword::In, ident)
                 } else if name == "with" {
                     Token::Keyword(Keyword::With, ident)
+                } else if name == "dyn" {
+                    Token::Keyword(Keyword::Dyn, ident)
                 } else {
                     Token::Ident(ident)
                 }
diff --git a/typed-html/Cargo.toml b/typed-html/Cargo.toml
index 15ecd06..d79cddd 100644
--- a/typed-html/Cargo.toml
+++ b/typed-html/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "typed-html"
 version = "0.2.2"
-edition = "2018"
+edition = "2021"
 authors = ["Bodil Stokke <bodil@bodil.org>"]
 license = "MPL-2.0+"
 description = "Type checked JSX for Rust"
diff --git a/typed-html/src/elements.rs b/typed-html/src/elements.rs
index 42369bf..682a213 100644
--- a/typed-html/src/elements.rs
+++ b/typed-html/src/elements.rs
@@ -54,8 +54,8 @@ declare_elements! {
     html {
         xmlns: Uri,
     } with [head, body];
-    head with [title] MetadataContent;
-    body with FlowContent;
+    head with [title] dyn MetadataContent;
+    body with dyn FlowContent;
 
     // Metadata
     base {
@@ -78,6 +78,7 @@ declare_elements! {
         content: String,
         http_equiv: HTTPEquiv,
         name: Metadata,
+        property: String, // This is non-standard, but used by the Open Graph protocol
     } in [MetadataContent];
     style {
         type: Mime,
@@ -96,11 +97,11 @@ declare_elements! {
         rel: SpacedList<LinkType>,
         target: Target,
         type: Mime,
-    } in [FlowContent, PhrasingContent, InteractiveContent] with FlowContent;
-    abbr in [FlowContent, PhrasingContent] with PhrasingContent;
-    address in [FlowContent] with FlowContent;
-    article in [FlowContent, SectioningContent] with FlowContent;
-    aside in [FlowContent, SectioningContent] with FlowContent;
+    } in [FlowContent, PhrasingContent, InteractiveContent] with dyn FlowContent;
+    abbr in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    address in [FlowContent] with dyn FlowContent;
+    article in [FlowContent, SectioningContent] with dyn FlowContent;
+    aside in [FlowContent, SectioningContent] with dyn FlowContent;
     audio {
         autoplay: Bool,
         controls: Bool,
@@ -109,13 +110,13 @@ declare_elements! {
         muted: Bool,
         preload: Preload,
         src: Uri,
-    } in [FlowContent, PhrasingContent, EmbeddedContent] with MediaContent;
-    b in [FlowContent, PhrasingContent] with PhrasingContent;
-    bdo in [FlowContent, PhrasingContent] with PhrasingContent;
-    bdi in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent, EmbeddedContent] with dyn MediaContent;
+    b in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    bdo in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    bdi in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     blockquote {
         cite: Uri,
-    } in [FlowContent] with FlowContent;
+    } in [FlowContent] with dyn FlowContent;
     br in [FlowContent, PhrasingContent];
     button {
         autofocus: Bool,
@@ -129,28 +130,28 @@ declare_elements! {
         name: Id,
         type: ButtonType,
         value: String,
-    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with dyn PhrasingContent;
     canvas {
         height: usize,
         width: usize,
-    } in [FlowContent, PhrasingContent, EmbeddedContent] with FlowContent;
-    cite in [FlowContent, PhrasingContent] with PhrasingContent;
-    code in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent, EmbeddedContent] with dyn FlowContent;
+    cite in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    code in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     data {
         value: String,
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     datalist in [FlowContent, PhrasingContent] with option;
     del {
         cite: Uri,
         datetime: Datetime,
-    } in [FlowContent, PhrasingContent] with FlowContent;
+    } in [FlowContent, PhrasingContent] with dyn FlowContent;
     details {
         open: Bool,
-    } in [FlowContent, SectioningContent, InteractiveContent] with [summary] FlowContent;
-    dfn in [FlowContent, PhrasingContent] with PhrasingContent;
-    div in [FlowContent] with FlowContent;
-    dl in [FlowContent] with DescriptionListContent;
-    em in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, SectioningContent, InteractiveContent] with [summary] dyn FlowContent;
+    dfn in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    div in [FlowContent] with dyn FlowContent;
+    dl in [FlowContent] with dyn DescriptionListContent;
+    em in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     embed {
         height: usize,
         src: Uri,
@@ -158,10 +159,10 @@ declare_elements! {
         width: usize,
     } in [FlowContent, PhrasingContent, EmbeddedContent, InteractiveContent];
     // FIXME the legend attribute should be optional
-    fieldset in [FlowContent, SectioningContent, FormContent] with [legend] FlowContent;
+    fieldset in [FlowContent, SectioningContent, FormContent] with [legend] dyn FlowContent;
     // FIXME the figcaption attribute should be optional
-    figure in [FlowContent, SectioningContent] with [figcaption] FlowContent;
-    footer in [FlowContent] with FlowContent;
+    figure in [FlowContent, SectioningContent] with [figcaption] dyn FlowContent;
+    footer in [FlowContent] with dyn FlowContent;
     form {
         accept-charset: SpacedList<CharacterEncoding>,
         action: Uri,
@@ -171,17 +172,17 @@ declare_elements! {
         name: Id,
         novalidate: Bool,
         target: Target,
-    } in [FlowContent] with FlowContent;
-    h1 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    h2 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    h3 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    h4 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    h5 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    h6 in [FlowContent, HeadingContent, HGroupContent] with PhrasingContent;
-    header in [FlowContent] with FlowContent;
-    hgroup in [FlowContent, HeadingContent] with HGroupContent;
+    } in [FlowContent] with dyn FlowContent;
+    h1 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    h2 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    h3 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    h4 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    h5 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    h6 in [FlowContent, HeadingContent, HGroupContent] with dyn PhrasingContent;
+    header in [FlowContent] with dyn FlowContent;
+    hgroup in [FlowContent, HeadingContent] with dyn HGroupContent;
     hr in [FlowContent];
-    i in [FlowContent, PhrasingContent] with PhrasingContent;
+    i in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     iframe {
         allow: FeaturePolicy,
         allowfullscreen: Bool,
@@ -193,7 +194,7 @@ declare_elements! {
         src: Uri,
         srcdoc: Uri,
         width: usize,
-    } in [FlowContent, PhrasingContent, EmbeddedContent, InteractiveContent] with FlowContent;
+    } in [FlowContent, PhrasingContent, EmbeddedContent, InteractiveContent] with dyn FlowContent;
     img {
         alt: String,
         crossorigin: CrossOrigin,
@@ -244,17 +245,17 @@ declare_elements! {
     ins {
         cite: Uri,
         datetime: Datetime,
-    } in [FlowContent, PhrasingContent] with FlowContent;
-    kbd in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn FlowContent;
+    kbd in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     label {
         for: Id,
         form: Id,
-    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with PhrasingContent;
-    main in [FlowContent] with FlowContent;
+    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with dyn PhrasingContent;
+    main in [FlowContent] with dyn FlowContent;
     map {
         name: Id,
-    } in [FlowContent, PhrasingContent] with MapContent;
-    mark in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn MapContent;
+    mark in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     // TODO the <math> element
     meter {
         value: isize,
@@ -264,9 +265,9 @@ declare_elements! {
         high: isize,
         optimum: isize,
         form: Id,
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
-    nav in [FlowContent, SectioningContent] with FlowContent;
-    noscript in [MetadataContent, FlowContent, PhrasingContent] with Node;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    nav in [FlowContent, SectioningContent] with dyn FlowContent;
+    noscript in [MetadataContent, FlowContent, PhrasingContent] with dyn Node;
     object {
         data: Uri,
         form: Id,
@@ -286,19 +287,19 @@ declare_elements! {
         for: SpacedSet<Id>,
         form: Id,
         name: Id,
-    } in [FlowContent, PhrasingContent, FormContent] with PhrasingContent;
-    p in [FlowContent] with PhrasingContent;
-    pre in [FlowContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent, FormContent] with dyn PhrasingContent;
+    p in [FlowContent] with dyn PhrasingContent;
+    pre in [FlowContent] with dyn PhrasingContent;
     progress {
         max: f64,
         value: f64,
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     q {
         cite: Uri,
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
-    ruby in [FlowContent, PhrasingContent] with PhrasingContent;
-    s in [FlowContent, PhrasingContent] with PhrasingContent;
-    samp in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    ruby in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    s in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    samp in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     script {
         async: Bool,
         crossorigin: CrossOrigin,
@@ -310,7 +311,7 @@ declare_elements! {
         text: String,
         type: String, // TODO could be an enum
     } in [MetadataContent, FlowContent, PhrasingContent, TableColumnContent] with TextNode;
-    section in [FlowContent, SectioningContent] with FlowContent;
+    section in [FlowContent, SectioningContent] with dyn FlowContent;
     select {
         autocomplete: String,
         autofocus: Bool,
@@ -320,14 +321,14 @@ declare_elements! {
         name: Id,
         required: Bool,
         size: usize,
-    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with SelectContent;
-    small in [FlowContent, PhrasingContent] with PhrasingContent;
-    span in [FlowContent, PhrasingContent] with PhrasingContent;
-    strong in [FlowContent, PhrasingContent] with PhrasingContent;
-    sub in [FlowContent, PhrasingContent] with PhrasingContent;
-    sup in [FlowContent, PhrasingContent] with PhrasingContent;
-    table in [FlowContent] with TableContent;
-    template in [MetadataContent, FlowContent, PhrasingContent, TableColumnContent] with Node;
+    } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with dyn SelectContent;
+    small in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    span in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    strong in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    sub in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    sup in [FlowContent, PhrasingContent] with dyn PhrasingContent;
+    table in [FlowContent] with dyn TableContent;
+    template in [MetadataContent, FlowContent, PhrasingContent, TableColumnContent] with dyn Node;
     textarea {
         autocomplete: OnOff,
         autofocus: Bool,
@@ -346,9 +347,9 @@ declare_elements! {
     } in [FlowContent, PhrasingContent, InteractiveContent, FormContent] with TextNode;
     time {
         datetime: Datetime,
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     ul in [FlowContent] with li;
-    var in [FlowContent, PhrasingContent] with PhrasingContent;
+    var in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     video {
         autoplay: Bool,
         controls: Bool,
@@ -361,7 +362,7 @@ declare_elements! {
         poster: Uri,
         src: Uri,
         width: usize,
-    } in [FlowContent, PhrasingContent, EmbeddedContent] with MediaContent;
+    } in [FlowContent, PhrasingContent, EmbeddedContent] with dyn MediaContent;
     wbr in [FlowContent, PhrasingContent];
 
     // Non-group elements
@@ -376,20 +377,20 @@ declare_elements! {
         shape: AreaShape,
         target: Target,
     } in [MapContent];
-    caption in [TableContent] with FlowContent;
+    caption in [TableContent] with dyn FlowContent;
     col {
         span: usize,
     };
     colgroup {
         span: usize,
     } in [TableContent] with col;
-    dd in [DescriptionListContent] with FlowContent;
-    dt in [DescriptionListContent] with FlowContent;
-    figcaption with FlowContent;
-    legend with PhrasingContent;
+    dd in [DescriptionListContent] with dyn FlowContent;
+    dt in [DescriptionListContent] with dyn FlowContent;
+    figcaption with dyn FlowContent;
+    legend with dyn PhrasingContent;
     li {
         value: isize,
-    } with FlowContent;
+    } with dyn FlowContent;
     option {
         disabled: Bool,
         label: String,
@@ -408,13 +409,13 @@ declare_elements! {
         src: Uri,
         type: Mime,
     } in [MediaContent];
-    summary with PhrasingContent;
+    summary with dyn PhrasingContent;
     tbody in [TableContent] with tr;
     td {
         colspan: usize,
         headers: SpacedSet<Id>,
         rowspan: usize,
-    } in [TableColumnContent] with FlowContent;
+    } in [TableColumnContent] with dyn FlowContent;
     tfoot in [TableContent] with tr;
     th {
         abbr: String,
@@ -422,9 +423,9 @@ declare_elements! {
         headers: SpacedSet<Id>,
         rowspan: usize,
         scope: TableHeaderScope,
-    } in [TableColumnContent] with FlowContent;
+    } in [TableColumnContent] with dyn FlowContent;
     thead in [TableContent] with tr;
-    tr in [TableContent] with TableColumnContent;
+    tr in [TableContent] with dyn TableColumnContent;
     track {
         default: Bool,
         kind: VideoKind,
@@ -434,7 +435,7 @@ declare_elements! {
     } in [MediaContent];
 
     // Don't @ me
-    blink in [FlowContent, PhrasingContent] with PhrasingContent;
+    blink in [FlowContent, PhrasingContent] with dyn PhrasingContent;
     marquee {
         behavior: String, // FIXME enum
         bgcolor: String, // FIXME colour
@@ -447,7 +448,7 @@ declare_elements! {
         truespeed: Bool,
         vspace: String, // FIXME size
         width: String, // FIXME size
-    } in [FlowContent, PhrasingContent] with PhrasingContent;
+    } in [FlowContent, PhrasingContent] with dyn PhrasingContent;
 }
 
 #[test]
diff --git a/typed-html/src/types/mod.rs b/typed-html/src/types/mod.rs
index 0fb5881..a92d7c7 100644
--- a/typed-html/src/types/mod.rs
+++ b/typed-html/src/types/mod.rs
@@ -28,6 +28,34 @@ pub type Integrity = String;
 pub type Nonce = String;
 pub type Target = String;
 
+pub trait DisplayAttribute {
+    fn write_attribute(
+        &self,
+        attr_str: &str,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> std::result::Result<(), std::fmt::Error>;
+
+    fn to_string_value(&self) -> String;
+}
+
+impl<Attribute: std::fmt::Display> DisplayAttribute for Attribute {
+    fn write_attribute(
+        &self,
+        attr_str: &str,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> std::result::Result<(), std::fmt::Error> {
+        let value = crate::escape_html_attribute(self.to_string());
+        if !value.is_empty() {
+            write!(f, " {}=\"{}\"", attr_str, value)?;
+        }
+        Ok(())
+    }
+
+    fn to_string_value(&self) -> String {
+        self.to_string()
+    }
+}
+
 #[derive(EnumString, Display, PartialEq, Eq, PartialOrd, Ord, AsRefStr, IntoStaticStr)]
 pub enum AreaShape {
     #[strum(to_string = "rect")]
@@ -60,11 +88,9 @@ pub enum ButtonType {
     Button,
 }
 
-#[derive(EnumString, Display, PartialEq, Eq, PartialOrd, Ord, AsRefStr, IntoStaticStr)]
+#[derive(EnumString, PartialEq, Eq, PartialOrd, Ord, AsRefStr, IntoStaticStr)]
 pub enum Bool {
-    #[strum(to_string = "true")]
     True,
-    #[strum(to_string = "")]
     False,
 }
 
@@ -78,6 +104,44 @@ impl From<bool> for Bool {
     }
 }
 
+impl DisplayAttribute for Bool {
+    fn write_attribute(
+        &self,
+        attr_str: &str,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> std::result::Result<(), std::fmt::Error> {
+        match self {
+            Bool::True => write!(f, " {}", attr_str),
+            Bool::False => Ok(()),
+        }
+    }
+
+    fn to_string_value(&self) -> String {
+        match self {
+            Bool::True => "true".to_string(),
+            Bool::False => "false".to_string(),
+        }
+    }
+}
+
+#[derive(EnumString, Display, PartialEq, Eq, PartialOrd, Ord, AsRefStr, IntoStaticStr)]
+pub enum EnumeratedBool {
+    #[strum(to_string = "true")]
+    True,
+    #[strum(to_string = "false")]
+    False,
+}
+
+impl From<bool> for EnumeratedBool {
+    fn from(v: bool) -> Self {
+        if v {
+            EnumeratedBool::True
+        } else {
+            EnumeratedBool::False
+        }
+    }
+}
+
 #[derive(EnumString, Display, PartialEq, Eq, PartialOrd, Ord, AsRefStr, IntoStaticStr)]
 pub enum CrossOrigin {
     #[strum(to_string = "anonymous")]
diff --git a/ui/Cargo.toml b/ui/Cargo.toml
index 84eb5c8..0c70069 100644
--- a/ui/Cargo.toml
+++ b/ui/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "typed-html-tests"
 version = "0.0.0"
-edition = "2018"
+edition = "2021"
 authors = ["Bodil Stokke <bodil@bodil.org>"]
 publish = false