Individual skill
doctype
Use when applies to all HTML documents. Always check the very first line of the raw HTML source (view-source: or DevTools > Network > Response). In SSR frameworks, check the root HTML template (e.g., _document.tsx in Next.js, index.html in Vite/Vue, application.html.erb in Rails). The doctype is not a tag — it is a declaration and does not need a closing tag.
skills/doctype/SKILL.md
Source commit e8d14d049010e2ce25afcd5e8609dfcbbe63e47e
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- Matches the observed repository HEAD
- Yes
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- Observed behavior
- Not checked yet
- Last checked
- Nov 2, 2026
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Repository context
- Repository stars (not skill ratings)
- 74,679
- Repository last updated
- Oct 31, 2026
- License
- MIT
Observed agent integration: No specific integration evidence. Standard SKILL.md installation does not imply verified compatibility.
- frontend Repository topic
- front-end-development Repository topic
- front-end-developer-tool Repository topic
- guidelines Repository topic
- checklist Repository topic
- reference Repository topic
- web-development Repository topic
- html Repository topic
- css Repository topic
- javascript Repository topic
- engineering Derived category
- engineering-web Derived category
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SKILL.mdcore · 3,334 bytesSKILL.mdcore · 3,334 bytesSKILL.mdcore · 3,334 bytesreferences/rule.mddoc · 3,204 bytes
Read excerpt
# Use the HTML5 doctype
Without the HTML5 doctype, browsers fall back to Quirks Mode, which emulates the rendering behavior of pre-standards browsers from the late 1990s. In Quirks Mode, the CSS box model changes (padding adds to element width), vertical centering breaks, and many modern HTML5 APIs may behave unexpectedly. This causes cross-browser rendering inconsistencies that are difficult to debug and may produce inaccessible layouts.
## Quick Reference
- Every HTML document must begin with `<!DOCTYPE html>` as its very first line
- Without a doctype, browsers enter Quirks Mode — a legacy compatibility mode with inconsistent CSS and JavaScript behavior
- The HTML5 doctype is case-insensitive but `<!DOCTYPE html>` (uppercase DOCTYPE) is the conventional form
- No other content, not even whitespace or BOM characters, should appear before the doctype
- Frameworks like Next.js, Nuxt, and Remix automatically include the doctype — verify in rendered HTML, not source templates
## Check
