HTML API: Introduce HTML Template Renderer

Currently only renders text data:

 - does not render nested HTML (escapes everything)
 - does not escape URLs

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -1983,8 +1983,8 @@ private function after_tag() {
 		$this->token_length         = null;
 		$this->tag_name_starts_at   = null;
 		$this->tag_name_length      = null;
-		$this->text_starts_at       = 0;
-		$this->text_length          = 0;
+		$this->text_starts_at       = null;
+		$this->text_length          = null;
 		$this->is_closing_tag       = null;
 		$this->attributes           = array();
 		$this->duplicate_attributes = null;

src/wp-includes/html-api/class-wp-html-template.php

@@ -0,0 +1,96 @@
+<?php
+
+/**
+ * WP_HTML_Template class.
+ *
+ * @since 6.5.0
+ */
+class WP_HTML_Template extends WP_HTML_Tag_Processor {
+	/**
+	 * Renders an HTML template, replacing the placeholders with the provided values.
+	 *
+	 * @since 6.5.0
+	 *
+	 * @param string $template The HTML template.
+	 * @param string $args     Array of key/value pairs providing substitue values for the placeholders.
+	 * @return string The rendered HTML.
+	 */
+	public static function render( $template, $args = array() ) {
+		$processor = new self( $template );
+		while ( $processor->next_token() ) {
+			$type = $processor->get_token_type();
+			$text = $processor->get_modifiable_text();
+
+			if ( '#funky-comment' === $type && strlen( $text ) > 0 && '%' === $text[0] ) {
+				$name  = substr( $text, 1 );
+				$value = isset( $args[ $name ] ) && is_string( $args[ $name ] ) ? $args[ $name ] : null;
+				$processor->set_bookmark( 'here' );
+				$processor->lexical_updates[] = new WP_HTML_Text_Replacement(
+					$processor->bookmarks['here']->start,
+					$processor->bookmarks['here']->length,
+					null === $value ? '' : esc_html( $value )
+				);
+			}
+
+			if ( '#tag' === $type ) {
+				foreach ( $processor->get_attribute_names_with_prefix( '' ) ?? array() as $attribute_name ) {
+					if ( str_starts_with( $attribute_name, '...' ) ) {
+						$spread_name = substr( $attribute_name, 3 );
+						if ( isset( $args[ $spread_name ] ) && is_array( $args[ $spread_name ] ) ) {
+							foreach ( $args[ $spread_name ] as $key => $value ) {
+								if ( true === $value || null === $value || is_string( $value ) ) {
+									$processor->set_attribute( $key, $value );
+								}
+							}
+						}
+						$processor->remove_attribute( $attribute_name );
+					}
+
+					$value = $processor->get_attribute( $attribute_name );
+
+					if ( ! is_string( $value ) ) {
+						continue;
+					}
+
+					$full_match = null;
+					if ( preg_match( '~^</%([^>]+)>$~', $value, $full_match ) ) {
+						$name = $full_match[1];
+
+						if ( array_key_exists( $name, $args ) ) {
+							$value = $args[ $name ];
+							if ( null === $value ) {
+								$processor->remove_attribute( $attribute_name );
+							} elseif ( true === $value ) {
+								$processor->set_attribute( $attribute_name, true );
+							} elseif ( is_string( $value ) ) {
+								$processor->set_attribute( $attribute_name, esc_attr( $args[ $name ] ) );
+							} else {
+								$processor->remove_attribute( $attribute_name );
+							}
+						} else {
+							$processor->remove_attribute( $attribute_name );
+						}
+
+						continue;
+					}
+
+					$new_value = preg_replace_callback(
+						'~</%([^>]+)>~',
+						static function ( $matches ) use ( $args ) {
+							return is_string( $args[ $matches[1] ] )
+								? esc_attr( $args[ $matches[1] ] )
+								: '';
+						},
+						$value
+					);
+
+					if ( $new_value !== $value ) {
+						$processor->set_attribute( $attribute_name, $new_value );
+					}
+				}
+			}
+		}
+
+		return $processor->get_updated_html();
+	}
+}

src/wp-settings.php

@@ -245,6 +245,7 @@
 require ABSPATH . WPINC . '/html-api/class-wp-html-token.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor-state.php';
 require ABSPATH . WPINC . '/html-api/class-wp-html-processor.php';
+require ABSPATH . WPINC . '/html-api/class-wp-html-template.php';
 require ABSPATH . WPINC . '/class-wp-http.php';
 require ABSPATH . WPINC . '/class-wp-http-streams.php';
 require ABSPATH . WPINC . '/class-wp-http-curl.php';

tests/phpunit/tests/html-api/wpHtmlTemplate.php

@@ -0,0 +1,41 @@
+<?php
+/**
+ * Unit tests covering WP_HTML_Template functionality.
+ *
+ * @package WordPress
+ * @subpackage HTML-API
+ *
+ * @since 6.5.0
+ *
+ * @group html-api
+ *
+ * @coversDefaultClass WP_HTML_Template
+ */
+
+class Tests_HtmlApi_WpHtmlTemplate extends WP_UnitTestCase {
+	/**
+	 * Demonstrates how to pass values into an HTML template.
+	 *
+	 * @ticket {TICKET_NUMBER}
+	 */
+	public function test_basic_render() {
+		$html = WP_HTML_Template::render(
+			'<div class="is-test </%class>" ...div-args inert="</%is_inert>">Just a </%count> test</div>',
+			array(
+				'count'    => '<strong>Hi <3</strong>',
+				'class'    => '5>4',
+				'is_inert' => 'inert',
+				'div-args' => array(
+					'class'    => 'hoover',
+					'disabled' => true,
+				),
+			)
+		);
+
+		$this->assertSame(
+			'<div disabled class="hoover"  inert="inert">Just a &lt;strong&gt;Hi &lt;3&lt;/strong&gt; test</div>',
+			$html,
+			'Failed to properly render template.'
+		);
+	}
+}