Rules

46 rules, grouped by where the thing lives rather than by how bad it is. Severity is a mutable field; the namespace in a ruleId is permanent, because suppression comments and CI baselines are written against it.

attr/

Attributes that outlived what they configured.

attr/table-presentationalDeprecated
align, bgcolor, border, cellpadding, cellspacing, frame, rules, summary and width on table are obsolete; browsers still apply the styling ones as low-priority hints, so move them to CSS and describe tables with caption.

document/

The document element and what the whole page declares about itself.

document/doctypeHarmful
The page has no doctype, or one other than <!doctype html>; a missing or legacy doctype can put the browser in quirks or limited-quirks mode, where CSS layout follows old compatibility rules.
document/html-langHarmful
An <html> without a non-empty lang leaves the page's language unknown, so screen readers guess how to pronounce it; WCAG 3.1.1, Level A.
document/main-multipleHarmful
A document may have only one main element without the hidden attribute; with two, skip links and landmark navigation land on one and the other is missed.

element/

Elements the HTML Standard lists as obsolete.

element/acronymDeprecated
acronym is obsolete; browsers already treat it exactly like abbr, so use abbr.
element/appletDeprecated
applet is obsolete and nothing can run it: no browser loads Java applets, and the JDK itself removed the Applet API; only the fallback text shows.
element/basefontDeprecated
basefont is obsolete and does nothing: only old Internet Explorer applied it, and the HTML Standard renders it as display:none. Set the font in CSS.
element/bgsoundDeprecated
bgsound was Internet Explorer's autoplaying background sound; it is obsolete, and the HTML Standard makes it an unknown element that plays nothing. Use audio with controls.
element/bigDeprecated
big is obsolete: it only makes text one size larger, with no meaning attached. Use CSS font-size, or the element that says why the text matters.
element/blinkDeprecated
blink is obsolete and no current browser blinks it; don't recreate the effect, because blinking content fails WCAG 2.2.2 Pause, Stop, Hide.
element/centerDeprecated
center is obsolete presentational markup for horizontal centering; do it in CSS with text-align: center for text and margin-inline: auto for blocks.
element/dirDeprecated
The dir element is obsolete; browsers already treat it exactly like ul, so use ul. The global dir attribute is unrelated and current.
element/fontDeprecated
font is obsolete presentational markup that still styles text through color, face and size hints; move the styling to CSS.
element/framesetDeprecated
frameset, frame and noframes are obsolete; a frameset page is several separate documents in one window, so use one document with iframe and CSS, or server-side includes.
element/isindexDeprecated
isindex is obsolete, and browsers dropped its search-form behaviour in 2016–17 over XSS concerns; it now renders nothing useful. Use a form with a text input.
element/marqueeDeprecated
marquee is obsolete, yet every browser still scrolls it; auto-scrolling text with no pause control fails WCAG 2.2.2 Pause, Stop, Hide.
element/nobrDeprecated
nobr is obsolete presentational markup that stops text wrapping; set white-space: nowrap in CSS instead.
element/noembedDeprecated
noembed is obsolete and its content is never displayed, not even when the embed fails; put fallback content inside an object element instead.
element/plaintextDeprecated
plaintext is obsolete and never ends: everything after it, including the page's closing tags, becomes text; serve a text/plain file or use pre and code with the content escaped.
element/plugin-embedDeprecated
This object or embed loads a Flash, Java or Silverlight plug-in; no current browser runs plug-ins and all three platforms are end-of-life, so the content never appears.
element/spacerDeprecated
spacer is obsolete and does nothing in any current browser; the gap it was meant to add is gone, so set it with CSS margin or gap.
element/strikeDeprecated
strike is obsolete: it draws a line through text but tells assistive technology nothing; use del for a removal or s for text that is no longer accurate.
element/ttDeprecated
tt is obsolete presentational markup for monospace text; use code, kbd, samp or var for what the text is, and CSS if you only want the font.
element/xmpDeprecated
xmp is obsolete; it shows markup as literal text but can't hold character references or its own end tag, so use pre and code with < and & escaped.

head/

The shape of the head itself — order, position, what must come first.

head/canonical-multipleHarmful
A page that declares more than one canonical URL has Google ignore all of them, so its duplicate URLs stop consolidating.
head/charset-positionHarmful
The encoding declaration must be fully inside the first 1024 bytes or the parser never sees it.
head/viewportHarmful
Without a viewport meta tag, mobile browsers lay the page out at a desktop width of about 980px and shrink it to fit.

link/

link/preload-asHarmful
A preload with no as has no destination, so the browser fetches nothing and the critical resource loads late, as if the hint were never written.
link/preload-font-crossoriginHarmful
Fonts are always fetched in CORS mode; a font preload without crossorigin doesn't match that request, so the font is downloaded twice.
link/apple-touch-icon-precomposedUnnecessary
A second declaration of the home-screen icon whose only effect, skipping iOS's gloss overlay, has been the default since iOS 7.
link/image-srcUnnecessary
A pre-Open Graph hint for social preview images that was never accepted as a link relation and is no longer read.
link/mask-iconUnnecessary
A Safari-only monochrome pinned-tab icon that was never a link type; it should not be used.
link/shortcut-iconUnnecessary
shortcut is not a link relation; browsers already parse rel="shortcut icon" as icon, so the token is inert.
link/sitemapUnnecessary
Search engines find sitemaps through robots.txt and their webmaster tools; none documents reading a link to one from HTML.

meta/

meta/http-equiv-refreshHarmful
http-equiv=refresh moves the reader on a timer, which WCAG names as a failure and the back button cannot undo.
meta/og-relative-urlHarmful
og:url, og:image, og:video and og:audio must be absolute http(s) URLs; a relative or scheme-less value isn't a URL to the crawlers that build link previews.
meta/viewport-user-scalableHarmful
A viewport with user-scalable=no or a maximum-scale below 2 stops people zooming to read, failing WCAG 1.4.4.
meta/apple-mobile-web-app-capableUnnecessary
Apple's pre-manifest switch for launching a Home Screen web app standalone; the manifest's display member replaces it.
meta/application-nameUnnecessary
A standard metadata name for the web app's name, superseded in practice by the manifest's name and short_name.
meta/http-equiv-x-ua-compatibleUnnecessary
X-UA-Compatible only ever controlled Internet Explorer document modes, and no shipping browser reads it.
meta/keywordsUnnecessary
A comma-separated topic list that no major search engine ranks on, and that the HTML Standard itself warns against.
meta/mobile-web-app-capableUnnecessary
Chrome's pre-manifest switch for launching a home screen shortcut as an app; the manifest's display member replaced it.
meta/msapplicationUnnecessary
Internet Explorer pinned-site and Start-tile metadata; nothing that still ships reads it.
meta/twitterUnnecessary
Twitter Card metadata; X has removed its Cards documentation, and every other preview consumer reads Open Graph.

script/

script/json-ld-syntaxHarmful
A <script type="application/ld+json"> whose contents don't parse as JSON is discarded whole, so none of its structured data reaches search engines.
script/type-javascript-mimeUnnecessary
A JavaScript MIME type in script[type] is the default; the spec says to omit the attribute.