Conformance Checking

Rules that validate HTML against the HTML Living Standard.

Rule	Default	Description
attr-duplication	error	Detect duplicate attributes
attr-value-quotes 🔧	warning	Detect unquoted attribute values
colspan-rowspan-range	error	Validate colspan (1-1000) and rowspan (0-65534) ranges
dfn-no-dfn-descendants	error	Disallow dfn nested inside another dfn
deprecated-attr	error	Detect deprecated attributes
deprecated-element	error	Detect deprecated elements
disallowed-element	off	Detect disallowed elements
doctype 🔧	error	Require DOCTYPE declaration
empty-title	error	Detect empty title element
end-tag	warning	Detect missing end tags
figcaption-position	warning	Require figcaption as first or last child of figure
form-dup-name	warning	Detect duplicate form control names
header-footer-nesting	error	Detect header/footer/main nesting inside header or footer
heading-levels	error	Detect skipped heading levels
id-duplication	error	Detect duplicate id values
input-attr-applicability	warning	Detect attributes that don't apply to the input type
invalid-attr	error	Detect attributes not in the spec
link-constraints	error	Validate link element attribute constraints
meta-constraints	error	Validate meta element attribute constraints
no-empty-track-label	warning	Disallow empty label on track elements
no-duplicate-base	error	Detect multiple base or title elements
no-duplicate-dt	error	Detect duplicate dt names in dl
no-duplicate-in-head	error	Detect duplicate charset/viewport/description meta
no-empty-palpable-content	warning	Detect empty palpable content elements
no-implicit-button-type	warning	Require explicit type on button
no-autoplay-media	warning	Detect autoplay without muted on video/audio
no-nested-forms	error	Detect nested form elements
no-nested-interactive	error	Detect interactive content inside a or button
no-non-scalable-viewport	error	Detect user-scalable=no in viewport meta
no-orphaned-end-tag	error	Detect unmatched closing tags
no-tabindex-on-dialog	error	Detect tabindex on dialog elements
obsolete-but-conforming	warning	Detect obsolete but conforming features
permitted-contents	error	Detect children not permitted by the spec
picture-structure	error	Validate picture element structure
placeholder-label-option	warning	Detect missing placeholder option in required select
require-datetime	error	Require datetime attribute on time element
require-meta-charset	off	Require meta charset declaration
required-attr	error	Detect missing required attributes
required-element	error	Detect missing required child elements
script-type	error	Validate script type attribute values
src-not-empty	error	Detect empty src or href attributes
summary-first-child	error	Require summary as first child of details
th-content-restrictions	error	Detect disallowed elements inside th
unique-main	error	Require at most one visible main element
valid-attr-value	error	Validate enumerated attribute values
valid-autocomplete	warning	Validate autocomplete attribute values
valid-id	error	Require valid id values (non-empty, no whitespace)
valid-rel	error	Validate rel attribute values
track-has-srclang	error	Require srclang on subtitles track
void-content	error	Detect content inside void elements
no-duplicate-track	error	Detect duplicate track elements with same kind/srclang/label
no-multiple-default-track	error	Detect multiple default tracks in same category
base-before-urls	warning	Require base href before URL-bearing elements
address-content-model	error	Detect heading/sectioning/header/footer inside address
link-rel-or-itemprop	error	Require rel or itemprop on link
base-has-href-or-target	error	Require href or target on base