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<?php |
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namespace ABlocks\Classes; |
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|
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if ( ! defined( 'ABSPATH' ) ) { |
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exit; |
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} |
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|
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/** |
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* Split combined page CSS into a "critical" subset (inlined in <head>) and a |
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* deferred remainder (loaded non-blocking). |
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* |
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* The split is by CSS PROPERTY, not by selector. A rule is critical if it sets |
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* anything that affects layout/box size (dimensions, spacing, position, flex/ |
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* grid, typography metrics, …); only rules that are purely cosmetic (color, |
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* background, shadow, radius, transition, filter, opacity) or interaction |
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* states (:hover/:focus/:active) and @keyframes are deferred. Because the |
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* deferred CSS changes nothing that occupies space, applying it after paint |
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* cannot cause a layout shift (CLS) — which the earlier selector-prefix split |
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* did, since it deferred per-block spacing/sizing. |
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* |
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* Parsing respects brace depth so nested at-rules (@media/@supports) stay whole. |
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* The heuristic is deliberately biased toward "critical": a rule we can't prove |
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* is cosmetic stays inlined (safe but slightly less deferral). |
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*/ |
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class CriticalCss { |
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|
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/** |
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* Property-name substrings that affect layout/box size. Over-inclusive on |
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* purpose (e.g. matches background-position) — false positives only cost a |
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* little deferral; a miss would cost CLS. |
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*/ |
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const LAYOUT_TOKENS = [ |
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'width', 'height', 'margin', 'padding', 'border', 'inset', 'top', 'bottom', 'left', 'right', |
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'display', 'position', 'float', 'clear', 'flex', 'grid', 'gap', 'align', 'justify', 'place', 'order', |
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'font', 'spacing', 'text-align', 'text-indent', 'text-transform', 'white-space', 'vertical-align', |
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'writing-mode', 'direction', 'box-sizing', 'column', 'overflow', 'list-style', 'aspect-ratio', 'tab-size', |
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]; |
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|
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/** |
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* @param string $css Minified combined CSS. |
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* @return array [ critical_css, rest_css ] |
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*/ |
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public static function split( $css ) { |
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$css = (string) $css; |
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if ( '' === trim( $css ) ) { |
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return [ '', '' ]; |
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} |
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|
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$critical = ''; |
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$rest = ''; |
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$len = strlen( $css ); |
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$i = 0; |
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$buffer = ''; |
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|
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while ( $i < $len ) { |
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$char = $css[ $i ]; |
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$buffer .= $char; |
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|
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if ( '{' === $char ) { |
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$prelude = trim( substr( $buffer, 0, -1 ) ); |
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$depth = 1; |
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$i++; |
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while ( $i < $len && $depth > 0 ) { |
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$c = $css[ $i ]; |
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$buffer .= $c; |
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if ( '{' === $c ) { |
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$depth++; |
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} elseif ( '}' === $c ) { |
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$depth--; |
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} |
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$i++; |
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} |
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|
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if ( self::is_critical( $prelude, $buffer ) ) { |
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$critical .= $buffer; |
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} else { |
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$rest .= $buffer; |
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} |
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$buffer = ''; |
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continue; |
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} |
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$i++; |
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} |
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$rest .= $buffer; // trailing text without a block (shouldn't happen) |
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|
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return [ $critical, $rest ]; |
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} |
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|
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private static function is_critical( $prelude, $rule_text ) { |
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if ( '' === $prelude ) { |
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return false; |
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} |
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|
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// At-rules. |
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if ( '@' === $prelude[0] ) { |
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if ( 0 === stripos( $prelude, '@keyframes' ) || 0 === stripos( $prelude, '@-webkit-keyframes' ) ) { |
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return false; // animations never affect initial layout |
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} |
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if ( 0 === stripos( $prelude, '@font-face' ) ) { |
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return true; // fonts affect text metrics |
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} |
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if ( 0 === stripos( $prelude, '@media' ) || 0 === stripos( $prelude, '@supports' ) ) { |
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return self::has_layout( $rule_text ); // critical only if it changes layout |
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} |
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return true; // @import/@charset/other — keep (safe) |
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} |
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|
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// Interaction states can't shift the initial layout — defer them. |
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if ( preg_match( '/:(hover|focus|active|focus-within|focus-visible)\b/i', $prelude ) ) { |
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return false; |
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} |
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|
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return self::has_layout( $rule_text ); |
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} |
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|
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/** |
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* True if the rule text declares any layout-affecting property. |
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*/ |
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private static function has_layout( $rule_text ) { |
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// Property names appear right after '{' or ';'. This avoids matching |
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// pseudo-selectors and colons inside values (url(), data:, http:). |
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if ( ! preg_match_all( '/[{;]\s*(-?[a-z][a-z-]*)\s*:/i', $rule_text, $m ) ) { |
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return false; |
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} |
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foreach ( $m[1] as $prop ) { |
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$prop = strtolower( $prop ); |
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// Props that merely contain a layout token but are purely cosmetic: |
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// border-radius / *-color / *-style (incl. list-style, outline-*). |
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if ( preg_match( '/(-radius|-color|-style)$/', $prop ) ) { |
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continue; |
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} |
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foreach ( self::LAYOUT_TOKENS as $token ) { |
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if ( false !== strpos( $prop, $token ) ) { |
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return true; |
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} |
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} |
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} |
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return false; |
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} |
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} |
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|
