Inspect raw HTML responses for a correct HTML5 doctype at the start of the document. Identify missing or outdated declarations and preceding content, then guide fixes that enable standards-mode rendering in browsers.
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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.
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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.
<!DOCTYPE html> as its very first line<!DOCTYPE html> (uppercase DOCTYPE) is the conventional formCheck the very first line of the HTML document source (not the DOM — the raw HTTP response). The document must begin with <!DOCTYPE html> (case-insensitive). Flag if: (1) the doctype is missing entirely; (2) an old HTML4 or XHTML doctype is used (contains PUBLIC and a DTD URL); (3) the doctype is not the absolute first content (whitespace or BOM before it triggers some browsers to enter Quirks Mode); (4) the doctype has incorrect syntax.
Add <!DOCTYPE html> as the absolute first line of the HTML document, before the <html> tag. Remove any old HTML4 or XHTML doctypes. Ensure no whitespace, comments, or BOM markers precede the doctype. For framework projects: verify in the root HTML template file, and confirm in the rendered HTML by viewing page source in the browser.
The DOCTYPE declaration tells the browser which version of HTML the document uses. <!DOCTYPE html> activates Standards Mode (also called Full Standards Mode) in all modern browsers, ensuring consistent rendering behavior. Without it, browsers enter Quirks Mode to emulate old IE behavior — a compatibility mode that changes the box model, float clearing, table rendering, and other CSS behaviors. The HTML5 doctype was intentionally made simple (just <!DOCTYPE html>) to replace the verbose HTML4/XHTML doctypes that required a full DTD URL.
Review templates, server-rendered HTML, and shared components that output markup related to Use the HTML5 doctype. Flag exact elements, attributes, and routes where the rendered HTML violates the rule.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
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Create and review custom 404 pages that help visitors recover from broken or moved links through search and navigation. Check the rendered page and HTTP response so missing content returns a proper 404 status.
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Audit or create a dedicated About page that explains a website’s purpose and identifies its people or organization. Review the rendered output and supporting information, including expertise and contact details, to find gaps.
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Review and add automated accessibility checks for components, pages, and continuous integration workflows. Use axe-based tools to detect structural and attribute issues, while identifying checks that still need keyboard or screen reader testing.
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Review sign-in, registration, multi-factor authentication, and account recovery for accessibility barriers. Find blocked paste, difficult code entry, and inaccessible backup methods, then suggest authentication paths that support password managers, autofill, or device approval.
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Review form errors, success messages, and live updates so screen reader users receive important notifications. Check the HTML delivered to the browser, announcement settings, display duration, and dismissal controls, then identify specific elements that need changes.
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Review HTML tables so screen reader users can understand how headers and data relate. Check header markup, table descriptions, and merged cells, then identify specific fixes and ways to verify them.
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