amp-content-styles-workaround.php

<?php
/**
 * Plugin Name: AMP Content Styles Workaround
 * Description: Temporary measure for dealing with external stylesheets and style elements in content.
 * Author: Weston Ruter, XWP
 */

add_action( 'amp_init', function() {

	// Version compare because this should be addressed by 1.0.
	if ( version_compare( AMP__VERSION, '1.0', '>=' ) ) {
		return;
	}

	add_filter( 'amp_content_sanitizers', function( $sanitizers ) {
		return array_merge(
			array(
				'My_AMP_Internalize_External_CSS_Sanitizer' => array(),
			),
			$sanitizers
		);
	} );

	class My_AMP_Internalize_External_CSS_Sanitizer extends AMP_Base_Sanitizer {

		public function sanitize() {
			$xpath      = new DOMXPath( $this->dom );
			$lower_case = 'translate( %s, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz" )'; // In XPath 2.0 this is lower-case().
			$predicates = array(
				sprintf( '( self::link and @href and %s = "stylesheet" )', sprintf( $lower_case, '@rel' ) ),
			);
			if ( ! current_theme_supports( 'amp' ) ) {
				$predicates[] = sprintf( '( self::style and ( not( @type ) or %s = "text/css" ) )', sprintf( $lower_case, '@type' ) );
			}

			$elements = array();
			foreach ( $xpath->query( '//*[ ' . implode( ' or ', $predicates ) . ' ]' ) as $element ) {
				$elements[] = $element;
			}

			$css = '';
			foreach ( $elements as $element ) {
				if ( 'style' === $element->nodeName ) {
					$css .= $element->textContent;
				} elseif ( 'link' === $element->nodeName ) {
					$href = $element->getAttribute( 'href' );
					if ( ! preg_match( '#^(https?:)?//#', $href, $matches ) ) {
						continue;
					}
					if ( empty( $matches[1] ) ) {
						$href = ( is_ssl() ? 'https' : 'http' ) . ':' . $href;
					}
					$cache_key = md5( $href );
					$contents  = get_transient( $cache_key );
					if ( false === $contents ) {
						$r = wp_remote_get( $href );
						if ( 200 !== wp_remote_retrieve_response_code( $r ) ) {
							$contents = new WP_Error( wp_remote_retrieve_response_code( $r ) );
						} else {
							$contents = wp_remote_retrieve_body( $r );
						}
						set_transient( $cache_key, $contents, MONTH_IN_SECONDS );
					}
					if ( ! is_wp_error( $contents ) ) {
						$css .= $contents;
					}
				}
				$element->parentNode->removeChild( $element );
			}

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

			if ( current_theme_supports( 'amp' ) ) {
				$style = $this->dom->createElement( 'style' );
				$style->setAttribute( 'type', 'text/css' );
				$style->appendChild( $this->dom->createTextNode( $css ) );
				$head = $this->dom->getElementsByTagName( 'head' )->item( 0 );
				if ( $head ) {
					$head->appendChild( $style );
				}
			} else {
				add_action( 'amp_post_template_css', function() use ( $css ) {
					echo $css;
				} );
			}
		}
	}
} );