Add basic HTML validation test

.github/workflows/test.yml

@@ -0,0 +1,23 @@
+name: CI
+
+on:
+  push:
+    paths:
+      - '**/*.html'
+      - '**/*.py'
+      - '.github/workflows/**'
+      - 'README.md'
+  pull_request:
+    paths:
+      - '**/*.html'
+      - '**/*.py'
+      - '.github/workflows/**'
+      - 'README.md'
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Validate HTML
+        run: python3 validate_html.py

README.md

@@ -1 +1,13 @@
 # solarsystem
+
+## Running tests
+
+A validation script ensures `index.html` parses correctly and that any local
+asset references exist. Run it with:
+
+```bash
+python3 validate_html.py
+```
+
+The same script runs automatically in CI when you open a pull request or push
+changes.

validate_html.py

@@ -0,0 +1,61 @@
+import os
+import sys
+from html.parser import HTMLParser
+
+errors = []
+
+
+def check_asset(path_str: str):
+    if path_str.startswith("http://") or path_str.startswith("https://"):
+        return
+    file_path = os.path.join(os.path.dirname(__file__), path_str)
+    if not os.path.exists(file_path):
+        errors.append(f"Missing asset: {path_str}")
+
+
+class Validator(HTMLParser):
+    void_elements = {
+        "area", "base", "br", "col", "embed", "hr", "img", "input", "link",
+        "meta", "param", "source", "track", "wbr",
+    }
+
+    def __init__(self):
+        super().__init__()
+        self.stack = []
+
+    def handle_starttag(self, tag, attrs):
+        if tag not in self.void_elements:
+            self.stack.append(tag)
+        if tag in {"script", "link"}:
+            for attr, value in attrs:
+                if (tag == "script" and attr == "src") or (tag == "link" and attr == "href"):
+                    check_asset(value)
+
+    def handle_endtag(self, tag):
+        if not self.stack:
+            errors.append(f"Unexpected closing tag: {tag}")
+            return
+        last = self.stack.pop()
+        if last != tag:
+            errors.append(f"Mismatched tag: expected </{last}> got </{tag}>")
+
+
+if __name__ == "__main__":
+    with open("index.html", "r", encoding="utf-8") as f:
+        content = f.read()
+
+    parser = Validator()
+    try:
+        parser.feed(content)
+        parser.close()
+    except Exception as e:
+        errors.append(str(e))
+
+    if parser.stack:
+        errors.append("Unclosed tags: " + ", ".join(parser.stack))
+
+    if errors:
+        for e in errors:
+            print(e, file=sys.stderr)
+        sys.exit(1)
+    print("index.html looks valid and all assets exist.")