Help:HTML

$htmlpairsStatic = [ # Tags that must be closed
	'b', 'bdi', 'del', 'i', 'ins', 'u', 'font', 'big', 'small', 'sub', 'sup', 'h1',
	'h2', 'h3', 'h4', 'h5', 'h6', 'cite', 'code', 'em', 's',
	'strike', 'strong', 'tt', 'var', 'div', 'center',
	'blockquote', 'ol', 'ul', 'dl', 'table', 'caption', 'pre',
	'ruby', 'rb', 'rp', 'rt', 'rtc', 'p', 'span', 'abbr', 'dfn',
	'kbd', 'samp', 'data', 'time', 'mark'
];
# These tags can be self-closed. For tags not also on
# $htmlsingleonly, a self-closed tag will be emitted as
# an empty element (open-tag/close-tag pair).
$htmlsingle = [
	'br', 'wbr', 'hr', 'li', 'dt', 'dd', 'meta', 'link'
];

# Elements that cannot have close tags. This is (not coincidentally)
# also the list of tags for which the HTML 5 parsing algorithm
# requires you to "acknowledge the token's self-closing flag", i.e.
# a self-closing tag like <br/> is not an HTML 5 parse error only
# for this list.
$htmlsingleonly = [
	'br', 'wbr', 'hr', 'meta', 'link'
];

$htmlnest = [ # Tags that can be nested--??
	'table', 'tr', 'td', 'th', 'div', 'blockquote', 'ol', 'ul',
	'li', 'dl', 'dt', 'dd', 'font', 'big', 'small', 'sub', 'sup', 'span',
	'var', 'kbd', 'samp', 'em', 'strong', 'q', 'ruby', 'bdo'
];
$tabletags = [ # Can only appear inside table, we will close them
	'td', 'th', 'tr',
];
$htmllist = [ # Tags used by list
	'ul', 'ol',
];
$listtags = [ # Tags that can appear in a list
	'li',
];

$htmlsingleallowed = array_unique( array_merge( $htmlsingle, $tabletags ) );
$htmlelementsStatic = array_unique( array_merge( $htmlsingle, $htmlpairsStatic, $htmlnest ) );

// For lookup efficiency flip each attributes array so the keys are
// the valid attributes.
$merge = static function ( $a, $b, $c = [] ) {
	return array_merge(
		$a,
		array_fill_keys( $b, true ),
		array_fill_keys( $c, true ) );
};
$common = $merge( [], [
	# HTML
	'id',
	'class',
	'style',
	'lang',
	'dir',
	'title',
	'tabindex',

	# WAI-ARIA
	'aria-describedby',
	'aria-flowto',
	'aria-hidden',
	'aria-label',
	'aria-labelledby',
	'aria-level',
	'aria-owns',
	'role',

	# RDFa
	# These attributes are specified in section 9 of
	# https://www.w3.org/TR/2008/REC-rdfa-syntax-20081014
	'about',
	'property',
	'resource',
	'datatype',
	'typeof',

	# Microdata. These are specified by
	# https://html.spec.whatwg.org/multipage/microdata.html#the-microdata-model
	'itemid',
	'itemprop',
	'itemref',
	'itemscope',
	'itemtype',
] );

$block = $merge( $common, [ 'align' ] );

$tablealign = [ 'align', 'valign' ];
$tablecell = [
	'abbr',
	'axis',
	'headers',
	'scope',
	'rowspan',
	'colspan',
	'nowrap', # deprecated
	'width', # deprecated
	'height', # deprecated
	'bgcolor', # deprecated
];

# Numbers refer to sections in HTML 4.01 standard describing the element.
# See: https://www.w3.org/TR/html4/
$allowed = [
	# 7.5.4
	'div'        => $block,
	'center'     => $common, # deprecated
	'span'       => $common,

	# 7.5.5
	'h1'         => $block,
	'h2'         => $block,
	'h3'         => $block,
	'h4'         => $block,
	'h5'         => $block,
	'h6'         => $block,

	# 7.5.6
	# address

	# 8.2.4
	'bdo'        => $common,

	# 9.2.1
	'em'         => $common,
	'strong'     => $common,
	'cite'       => $common,
	'dfn'        => $common,
	'code'       => $common,
	'samp'       => $common,
	'kbd'        => $common,
	'var'        => $common,
	'abbr'       => $common,
	# acronym

	# 9.2.2
	'blockquote' => $merge( $common, [ 'cite' ] ),
	'q'          => $merge( $common, [ 'cite' ] ),

	# 9.2.3
	'sub'        => $common,
	'sup'        => $common,

	# 9.3.1
	'p'          => $block,

	# 9.3.2
	'br'         => $merge( $common, [ 'clear' ] ),

	# https://www.w3.org/TR/html5/text-level-semantics.html#the-wbr-element
	'wbr'        => $common,

	# 9.3.4
	'pre'        => $merge( $common, [ 'width' ] ),

	# 9.4
	'ins'        => $merge( $common, [ 'cite', 'datetime' ] ),
	'del'        => $merge( $common, [ 'cite', 'datetime' ] ),

	# 10.2
	'ul'         => $merge( $common, [ 'type' ] ),
	'ol'         => $merge( $common, [ 'type', 'start', 'reversed' ] ),
	'li'         => $merge( $common, [ 'type', 'value' ] ),

	# 10.3
	'dl'         => $common,
	'dd'         => $common,
	'dt'         => $common,

	# 11.2.1
	'table'      => $merge( $common,
						[ 'summary', 'width', 'border', 'frame',
								'rules', 'cellspacing', 'cellpadding',
								'align', 'bgcolor',
						] ),

	# 11.2.2
	'caption'    => $block,

	# 11.2.3
	'thead'      => $common,
	'tfoot'      => $common,
	'tbody'      => $common,

	# 11.2.4
	'colgroup'   => $merge( $common, [ 'span' ] ),
	'col'        => $merge( $common, [ 'span' ] ),

	# 11.2.5
	'tr'         => $merge( $common, [ 'bgcolor' ], $tablealign ),

	# 11.2.6
	'td'         => $merge( $common, $tablecell, $tablealign ),
	'th'         => $merge( $common, $tablecell, $tablealign ),

	# 12.2
	# NOTE: <a> is not allowed directly, but this list of allowed
	# attributes is used from the Parser object
	'a'          => $merge( $common, [ 'href', 'rel', 'rev' ] ), # rel/rev esp. for RDFa

	# 13.2
	# Not usually allowed, but may be used for extension-style hooks
	# such as <math> when it is rasterized
	'img'        => $merge( $common, [ 'alt', 'src', 'width', 'height', 'srcset' ] ),
	# Attributes for A/V tags added in T163583 / T133673
	'audio'      => $merge( $common, [ 'controls', 'preload', 'width', 'height' ] ),
	'video'      => $merge( $common, [ 'poster', 'controls', 'preload', 'width', 'height' ] ),
	'source'     => $merge( $common, [ 'type', 'src' ] ),
	'track'      => $merge( $common, [ 'type', 'src', 'srclang', 'kind', 'label' ] ),

	# 15.2.1
	'tt'         => $common,
	'b'          => $common,
	'i'          => $common,
	'big'        => $common,
	'small'      => $common,
	'strike'     => $common,
	's'          => $common,
	'u'          => $common,

	# 15.2.2
	'font'       => $merge( $common, [ 'size', 'color', 'face' ] ),
	# basefont

	# 15.3
	'hr'         => $merge( $common, [ 'width' ] ),

	# HTML Ruby annotation text module, simple ruby only.
	# https://www.w3.org/TR/html5/text-level-semantics.html#the-ruby-element
	'ruby'       => $common,
	# rbc
	'rb'         => $common,
	'rp'         => $common,
	'rt'         => $common, # $merge( $common, [ 'rbspan' ] ),
	'rtc'        => $common,

	# MathML root element, where used for extensions
	# 'title' may not be 100% valid here; it's XHTML
	# https://www.w3.org/TR/REC-MathML/
	'math'       => $merge( [], [ 'class', 'style', 'id', 'title' ] ),

	// HTML 5 section 4.5
	'figure'     => $common,
	'figcaption' => $common,

	# HTML 5 section 4.6
	'bdi' => $common,

	# HTML5 elements, defined by:
	# https://html.spec.whatwg.org/multipage/semantics.html#the-data-element
	'data' => $merge( $common, [ 'value' ] ),
	'time' => $merge( $common, [ 'datetime' ] ),
	'mark' => $common,

	// meta and link are only permitted by internalRemoveHtmlTags when Microdata
	// is enabled so we don't bother adding a conditional to hide these
	// Also meta and link are only valid in WikiText as Microdata elements
	// (ie: validateTag rejects tags missing the attributes needed for Microdata)
	// So we don't bother including $common attributes that have no purpose.
	'meta' => $merge( [], [ 'itemprop', 'content' ] ),
	'link' => $merge( [], [ 'itemprop', 'href', 'title' ] ),

	# HTML 5 section 4.3.5
	'aside' => $common,
];