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 framewor
复制下面这句话，粘贴给 Claude Code、Codex、Cursor 等 AI 编程工具，它会读取安装说明并在你确认后完成安装。
请阅读 https://ai.atlankj.com/install/asset/gh-doctype-dd27ccb9a179 ，按照其中的说明把「doctype」安装到你（当前 AI 工具）中。执行前先告诉我将运行的命令和写入的位置，等我确认。
查看 AI 将读取的安装说明正在读取 GitHub 原文…
内容来自 GitHub 原始文件，由原作者维护。在 GitHub 查看
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.
Rule page: https://frontendchecklist.io/en/rules/html/doctype