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function safecss_filter_attr( $css, $deprecated = '' ) {
	if ( ! empty( $deprecated ) ) {
		_deprecated_argument( __FUNCTION__, '2.8.1' ); // Never implemented.
	}

	$css = wp_kses_no_null( $css );
	$css = str_replace( array( "\n", "\r", "\t" ), '', $css );

	$allowed_protocols = wp_allowed_protocols();

	$css_array = explode( ';', trim( $css ) );

	/**
	 * Filters the list of allowed CSS attributes.
	 *
	 * @since 2.8.1
	 *
	 * @param string[] $attr Array of allowed CSS attributes.
	 */
	$allowed_attr = apply_filters(
		'safe_style_css',
		array(
			'background',
			'background-color',
			'background-image',
			'background-position',
			'background-repeat',
			'background-size',
			'background-attachment',
			'background-blend-mode',

			'border',
			'border-radius',
			'border-width',
			'border-color',
			'border-style',
			'border-right',
			'border-right-color',
			'border-right-style',
			'border-right-width',
			'border-bottom',
			'border-bottom-color',
			'border-bottom-left-radius',
			'border-bottom-right-radius',
			'border-bottom-style',
			'border-bottom-width',
			'border-bottom-right-radius',
			'border-bottom-left-radius',
			'border-left',
			'border-left-color',
			'border-left-style',
			'border-left-width',
			'border-top',
			'border-top-color',
			'border-top-left-radius',
			'border-top-right-radius',
			'border-top-style',
			'border-top-width',
			'border-top-left-radius',
			'border-top-right-radius',

			'border-spacing',
			'border-collapse',
			'caption-side',

			'columns',
			'column-count',
			'column-fill',
			'column-gap',
			'column-rule',
			'column-span',
			'column-width',

			'color',
			'filter',
			'font',
			'font-family',
			'font-size',
			'font-style',
			'font-variant',
			'font-weight',
			'letter-spacing',
			'line-height',
			'text-align',
			'text-decoration',
			'text-indent',
			'text-transform',

			'height',
			'min-height',
			'max-height',

			'width',
			'min-width',
			'max-width',

			'margin',
			'margin-right',
			'margin-bottom',
			'margin-left',
			'margin-top',
			'margin-block-start',
			'margin-block-end',
			'margin-inline-start',
			'margin-inline-end',

			'padding',
			'padding-right',
			'padding-bottom',
			'padding-left',
			'padding-top',
			'padding-block-start',
			'padding-block-end',
			'padding-inline-start',
			'padding-inline-end',

			'flex',
			'flex-basis',
			'flex-direction',
			'flex-flow',
			'flex-grow',
			'flex-shrink',
			'flex-wrap',

			'gap',
			'column-gap',
			'row-gap',

			'grid-template-columns',
			'grid-auto-columns',
			'grid-column-start',
			'grid-column-end',
			'grid-column',
			'grid-column-gap',
			'grid-template-rows',
			'grid-auto-rows',
			'grid-row-start',
			'grid-row-end',
			'grid-row',
			'grid-row-gap',
			'grid-gap',

			'justify-content',
			'justify-items',
			'justify-self',
			'align-content',
			'align-items',
			'align-self',

			'clear',
			'cursor',
			'direction',
			'float',
			'list-style-type',
			'object-fit',
			'object-position',
			'opacity',
			'overflow',
			'vertical-align',
			'writing-mode',

			'position',
			'top',
			'right',
			'bottom',
			'left',
			'z-index',
			'box-shadow',
			'aspect-ratio',
			'container-type',

			// Custom CSS properties.
			'--*',
		)
	);

	/*
	 * CSS attributes that accept URL data types.
	 *
	 * This is in accordance to the CSS spec and unrelated to
	 * the sub-set of supported attributes above.
	 *
	 * See: https://developer.mozilla.org/en-US/docs/Web/CSS/url
	 */
	$css_url_data_types = array(
		'background',
		'background-image',

		'cursor',
		'filter',

		'list-style',
		'list-style-image',
	);

	/*
	 * CSS attributes that accept gradient data types.
	 *
	 */
	$css_gradient_data_types = array(
		'background',
		'background-image',
	);

	if ( empty( $allowed_attr ) ) {
		return $css;
	}

	$css = '';
	foreach ( $css_array as $css_item ) {
		if ( '' === $css_item ) {
			continue;
		}

		$css_item        = trim( $css_item );
		$css_test_string = $css_item;
		$found           = false;
		$url_attr        = false;
		$gradient_attr   = false;
		$is_custom_var   = false;

		if ( ! str_contains( $css_item, ':' ) ) {
			$found = true;
		} else {
			$parts        = explode( ':', $css_item, 2 );
			$css_selector = trim( $parts[0] );

			// Allow assigning values to CSS variables.
			if ( in_array( '--*', $allowed_attr, true ) && preg_match( '/^--[a-zA-Z0-9-_]+$/', $css_selector ) ) {
				$allowed_attr[] = $css_selector;
				$is_custom_var  = true;
			}

			if ( in_array( $css_selector, $allowed_attr, true ) ) {
				$found         = true;
				$url_attr      = in_array( $css_selector, $css_url_data_types, true );
				$gradient_attr = in_array( $css_selector, $css_gradient_data_types, true );
			}

			if ( $is_custom_var ) {
				$css_value     = trim( $parts[1] );
				$url_attr      = str_starts_with( $css_value, 'url(' );
				$gradient_attr = str_contains( $css_value, '-gradient(' );
			}
		}

		if ( $found && $url_attr ) {
			// Simplified: matches the sequence `url(*)`.
			preg_match_all( '/url\([^)]+\)/', $parts[1], $url_matches );

			foreach ( $url_matches[0] as $url_match ) {
				// Clean up the URL from each of the matches above.
				preg_match( '/^url\(\s*([\'\"]?)(.*)(\g1)\s*\)$/', $url_match, $url_pieces );

				if ( empty( $url_pieces[2] ) ) {
					$found = false;
					break;
				}

				$url = trim( $url_pieces[2] );

				if ( empty( $url ) || wp_kses_bad_protocol( $url, $allowed_protocols ) !== $url ) {
					$found = false;
					break;
				} else {
					// Remove the whole `url(*)` bit that was matched above from the CSS.
					$css_test_string = str_replace( $url_match, '', $css_test_string );
				}
			}
		}

		if ( $found && $gradient_attr ) {
			$css_value = trim( $parts[1] );
			if ( preg_match( '/^(repeating-)?(linear|radial|conic)-gradient\(([^()]|rgb[a]?\([^()]*\))*\)$/', $css_value ) ) {
				// Remove the whole `gradient` bit that was matched above from the CSS.
				$css_test_string = str_replace( $css_value, '', $css_test_string );
			}
		}

		if ( $found ) {
			/*
			 * Allow CSS functions like var(), calc(), etc. by removing them from the test string.
			 * Nested functions and parentheses are also removed, so long as the parentheses are balanced.
			 */
			$css_test_string = preg_replace(
				'/\b(?:var|calc|min|max|minmax|clamp|repeat)(\((?:[^()]|(?1))*\))/',
				'',
				$css_test_string
			);

			/*
			 * Disallow CSS containing \ ( & } = or comments, except for within url(), var(), calc(), etc.
			 * which were removed from the test string above.
			 */
			$allow_css = ! preg_match( '%[\\\(&=}]|/\*%', $css_test_string );

			/**
			 * Filters the check for unsafe CSS in `safecss_filter_attr`.
			 *
			 * Enables developers to determine whether a section of CSS should be allowed or discarded.
			 * By default, the value will be false if the part contains \ ( & } = or comments.
			 * Return true to allow the CSS part to be included in the output.
			 *
			 * @since 5.5.0
			 *
			 * @param bool   $allow_css       Whether the CSS in the test string is considered safe.
			 * @param string $css_test_string The CSS string to test.
			 */
			$allow_css = apply_filters( 'safecss_filter_attr_allow_css', $allow_css, $css_test_string );

			// Only add the CSS part if it passes the regex check.
			if ( $allow_css ) {
				if ( '' !== $css ) {
					$css .= ';';
				}

				$css .= $css_item;
			}
		}
	}

	return $css;
}