diff --git a/html-api-debugger/html-api-debugger.php b/html-api-debugger/html-api-debugger.php
index 6a4a3f0..6aaefe3 100644
--- a/html-api-debugger/html-api-debugger.php
+++ b/html-api-debugger/html-api-debugger.php
@@ -45,6 +45,7 @@ function () {
 						$html = $request->get_json_params()['html'] ?: '';
 						$options = array(
 							'context_html' => $request->get_json_params()['contextHTML'] ?: null,
+							'selector' => $request->get_json_params()['selector'] ?: null,
 						);
 						return prepare_html_result_object( $html, $options );
 					},
@@ -112,6 +113,7 @@ function () {
 
 					$options = array(
 						'context_html' => null,
+						'selector' => null,
 					);
 
 					$html = '';
@@ -122,6 +124,9 @@ function () {
 					if ( isset( $_GET['contextHTML'] ) && is_string( $_GET['contextHTML'] ) ) {
 						$options['context_html'] = stripslashes( $_GET['contextHTML'] );
 					}
+					if ( isset( $_GET['selector'] ) && is_string( $_GET['selector'] ) ) {
+						$options['selector'] = stripslashes( $_GET['selector'] );
+					}
 					// phpcs:enable WordPress.Security.NonceVerification.Recommended
 
 					// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
diff --git a/html-api-debugger/html-api-integration.php b/html-api-debugger/html-api-integration.php
index 0875ecb..c9a4410 100644
--- a/html-api-debugger/html-api-integration.php
+++ b/html-api-debugger/html-api-integration.php
@@ -12,6 +12,9 @@
 function get_supports(): array {
 	return array(
 		'create_fragment_advanced' => method_exists( WP_HTML_Processor::class, 'create_fragment_at_current_node' ),
+		'selectors' =>
+			class_exists( '\WP_CSS_Complex_Selector_List' )
+			|| class_exists( '\WP_CSS_Compound_Selector_List' ),
 	);
 }
 
@@ -70,6 +73,25 @@ function get_normalized_html( string $html, array $options ): ?string {
  * @param array  $options The options.
  */
 function get_tree( string $html, array $options ): array {
+	/**
+	 * Messages generated during parse.
+	 *
+	 * @var string[]
+	 */
+	$warnings = array();
+	$selector = null;
+	if ( isset( $options['selector'] ) && class_exists( '\WP_CSS_Complex_Selector_List' ) ) {
+		$selector = \WP_CSS_Complex_Selector_List::from_selectors( $options['selector'] );
+		if ( null === $selector ) {
+			$warnings[] = 'The provided selector is invalid or unsupported.';
+		}
+	} elseif ( isset( $options['selector'] ) && class_exists( '\WP_CSS_Compound_Selector_List' ) ) {
+		$selector = \WP_CSS_Compound_Selector_List::from_selectors( $options['selector'] );
+		if ( null === $selector ) {
+			$warnings[] = 'The provided selector is invalid or unsupported.';
+		}
+	}
+
 	$processor_state = new ReflectionProperty( WP_HTML_Processor::class, 'state' );
 	$processor_state->setAccessible( true );
 
@@ -225,6 +247,8 @@ function get_tree( string $html, array $options ): array {
 					$document_title = $processor->get_modifiable_text();
 				}
 
+				$matches = $selector !== null && $selector->matches( $processor );
+
 				$attributes      = array();
 				$attribute_names = $processor->get_attribute_names_with_prefix( '' );
 				if ( null !== $attribute_names ) {
@@ -261,6 +285,7 @@ function get_tree( string $html, array $options ): array {
 					'_virtual' => $is_virtual(),
 					'_depth' => $processor->get_current_depth(),
 					'_namespace' => $namespace,
+					'_matches' => $matches,
 				);
 
 				// Self-contained tags contain their inner contents as modifiable text.
@@ -440,6 +465,7 @@ function get_tree( string $html, array $options ): array {
 		'doctypeSystemId' => $doctype_system_identifier,
 
 		'contextNode' => $context_node,
+		'warnings' => $warnings,
 	);
 }
 
diff --git a/html-api-debugger/interactivity.php b/html-api-debugger/interactivity.php
index 4fb7e43..ae03226 100644
--- a/html-api-debugger/interactivity.php
+++ b/html-api-debugger/interactivity.php
@@ -37,6 +37,7 @@ function generate_page( string $html, array $options ): string {
 			'showInvisible' => false,
 			'showVirtual' => false,
 			'contextHTML' => $options['context_html'] ?? '',
+			'selector' => $options['selector'] ?? '',
 
 			'hoverInfo' => 'breadcrumbs',
 			'hoverBreadcrumbs' => true,
@@ -47,6 +48,7 @@ function generate_page( string $html, array $options ): string {
 			'htmlApiDoctypeName' => $htmlapi_response['result']['doctypeName'] ?? null,
 			'htmlApiDoctypePublicId' => $htmlapi_response['result']['doctypePublicId'] ?? null,
 			'htmlApiDoctypeSytemId' => $htmlapi_response['result']['doctypeSystemId'] ?? null,
+			'treeWarnings' => $htmlapi_response['result']['warnings'] ?? array(),
 			'normalizedHtml' => $htmlapi_response['normalizedHtml'] ?? '',
 
 			'playbackLength' => isset( $htmlapi_response['result']['playback'] )
@@ -63,6 +65,17 @@ function generate_page( string $html, array $options ): string {
 	data-wp-init="run"
 	class="html-api-debugger-container html-api-debugger--grid"
 >
+	<div data-wp-bind--hidden="!state.htmlapiResponse.supports.create_fragment_advanced" class="full-width">
+		<label>Context in which input HTML finds itself
+			<textarea
+				class="context-html"
+				placeholder="Provide a fragment context, for example:&#x0A;<!DOCTYPE html><body>"
+				title="Leave blank to parse a full document."
+				rows="2"
+				data-wp-on-async--input="handleContextHtmlInput"
+			><?php echo "\n" . esc_textarea( str_replace( "\0", '', $options['context_html'] ?? '' ) ); ?></textarea>
+		</label>
+	</div>
 	<div>
 		<h2>Input HTML</h2>
 		<textarea
@@ -139,15 +152,8 @@ class="html-api-debugger-container html-api-debugger--grid"
 				<label>Show closers <input type="checkbox" data-wp-bind--checked="state.showClosers" data-wp-on-async--input="handleShowClosersClick"></label>
 				<label>Show invisible <input type="checkbox" data-wp-bind--checked="state.showInvisible" data-wp-on-async--input="handleShowInvisibleClick"></label>
 				<span><label>Show virtual <input type="checkbox" data-wp-bind--checked="state.showVirtual" data-wp-on-async--input="handleShowVirtualClick"></label></span>
-				<div data-wp-bind--hidden="!state.htmlapiResponse.supports.create_fragment_advanced">
-					<label>Context html
-						<textarea
-							class="context-html"
-							placeholder="Provide a fragment context, for example:&#x0A;<!DOCTYPE html><body>"
-							rows="2"
-							data-wp-on-async--input="handleContextHtmlInput"
-						><?php echo "\n" . esc_textarea( str_replace( "\0", '', $options['context_html'] ?? '' ) ); ?></textarea>
-				</label>
+				<div data-wp-bind--hidden="!state.htmlapiResponse.supports.selectors">
+					<label>CSS Selectors <textarea placeholder="CSS selector: .my-class" data-wp-on-async--input="handleSelectorChange"><?php echo "\n" . esc_textarea( str_replace( "\0", '', $options['selector'] ?? '' ) ); ?></textarea></label>
 				</div>
 			</div>
 			<div>
@@ -161,6 +167,13 @@ class="context-html"
 			</div>
 		</div>
 
+		<div data-wp-bind--hidden="!state.treeWarnings.length">
+			<template data-wp-each="state.treeWarnings">
+				<p data-wp-text="context.item" class="error-holder"></p>
+			</template>
+		</div>
+		<p data-wp-bind--hidden="!state.selectorErrorMessage" data-wp-text="state.selectorErrorMessage" class="error-holder"></p>
+
 		<div>
 			<h2>Processed HTML</h2>
 			<div data-wp-bind--hidden="!state.htmlapiResponse.result.playback">
diff --git a/html-api-debugger/print-html-tree.js b/html-api-debugger/print-html-tree.js
index 378150c..2ba33ec 100644
--- a/html-api-debugger/print-html-tree.js
+++ b/html-api-debugger/print-html-tree.js
@@ -5,6 +5,7 @@ import { replaceInvisible } from '@html-api-debugger/replace-invisible-chars';
  * @property {boolean} [showClosers]
  * @property {boolean} [showInvisible]
  * @property {boolean} [showVirtual]
+ * @property {string|null} [selector]
  * @property {'breadcrumbs'|'insertionMode'} [hoverInfo]
  */
 
@@ -21,6 +22,14 @@ export function printHtmlApiTree(node, ul, options = {}) {
 	for (let i = 0; i < node.childNodes.length; i += 1) {
 		const li = document.createElement('li');
 		li.className = `t${node.childNodes[i].nodeType}`;
+
+		if (
+			node.childNodes[i]._matches ||
+			(options.selector && node.childNodes[i].matches?.(options.selector))
+		) {
+			li.classList.add('matches-selector');
+		}
+
 		if (node.childNodes[i].nodeType === Node.prototype.DOCUMENT_TYPE_NODE) {
 			li.appendChild(document.createTextNode('DOCTYPE: '));
 		}
diff --git a/html-api-debugger/style.css b/html-api-debugger/style.css
index 9094aed..433723e 100644
--- a/html-api-debugger/style.css
+++ b/html-api-debugger/style.css
@@ -7,6 +7,9 @@
 }
 
 .html-api-debugger-container {
+	--monospace-font-family: ui-monospace, SFMono-Regular, SF Mono, Menlo,
+		Consolas, Liberation Mono, monospace;
+
 	width: 100%;
 	padding: 20px 20px 0 0;
 
@@ -32,6 +35,15 @@
 		grid-column: 1 / -1;
 	}
 
+	.matches-selector {
+		outline: 1px dotted hotpink;
+	}
+
+	code,
+	pre {
+		font-family: var(--monospace-font-family);
+	}
+
 	pre {
 		background-color: #fff;
 		border: inset 1px;
@@ -49,12 +61,12 @@
 	#input_html {
 		width: 100%;
 		min-height: 200px;
-		font-family: monospace;
+		font-family: var(--monospace-font-family);
 	}
 
 	.context-html {
 		width: 100%;
-		font-family: monospace;
+		font-family: var(--monospace-font-family);
 
 		&:placeholder-shown {
 			font-style: italic;
@@ -142,7 +154,7 @@
 	border: inset 1px;
 	padding: 0.5em 0.5em 0.5em 1em;
 	color: black;
-	font-family: monospace;
+	font-family: var(--monospace-font-family);
 	background: white;
 	margin: 0;
 
@@ -165,7 +177,7 @@
 
 	.t2 {
 		font-style: normal;
-		font-family: monospace;
+		font-family: var(--monospace-font-family);
 	}
 
 	.t2 .name {
diff --git a/html-api-debugger/view.js b/html-api-debugger/view.js
index c3345c0..88c1ff9 100644
--- a/html-api-debugger/view.js
+++ b/html-api-debugger/view.js
@@ -49,6 +49,7 @@ let mutationObserver = null;
  *
  * @typedef Supports
  * @property {boolean} create_fragment_advanced
+ * @property {boolean} selectors
  *
  *
  * @typedef HtmlApiResponse
@@ -67,6 +68,9 @@ let mutationObserver = null;
  *
  *
  * @typedef  State
+ * @property {ReadonlyArray<string>} treeWarnings
+ * @property {string|null} selector
+ * @property {string|null} selectorErrorMessage
  * @property {boolean} showClosers
  * @property {boolean} showInvisible
  * @property {boolean} showVirtual
@@ -139,9 +143,16 @@ const store = createStore(NS, {
 				showInvisible: store.state.showInvisible,
 				showVirtual: store.state.showVirtual,
 				hoverInfo: store.state.hoverInfo,
+				selector: store.state.htmlapiResponse.supports.selectors
+					? store.state.selector
+					: '',
 			};
 		},
 
+		get treeWarnings() {
+			return store.state.htmlapiResponse.result?.warnings ?? [];
+		},
+
 		get playbackTree() {
 			if (store.state.playbackPoint === null) {
 				return undefined;
@@ -253,6 +264,9 @@ const store = createStore(NS, {
 			if (store.state.contextHTMLForUse) {
 				searchParams.set('contextHTML', store.state.contextHTMLForUse);
 			}
+			if (store.state.selector) {
+				searchParams.set('selector', store.state.selector);
+			}
 			const base = '/wp-admin/admin.php';
 			const u = new URL(
 				'https://playground.wordpress.net/?plugin=html-api-debugger',
@@ -396,10 +410,23 @@ const store = createStore(NS, {
 		/** @type {Element|null} */
 		let contextElement = null;
 		if (store.state.contextHTMLForUse) {
-			const walker = doc.createTreeWalker(doc, NodeFilter.SHOW_ELEMENT);
-			while (walker.nextNode()) {
-				// @ts-expect-error It's an Element!
-				contextElement = walker.currentNode;
+			// An HTML document will always make HTML > HEAD + BODY.
+			// But that may not be the intended context.
+			// Guess the intended context in case the HEAD and BODY elements are empty.
+			if (doc.body.hasChildNodes() || doc.head.hasChildNodes()) {
+				const walker = doc.createTreeWalker(doc, NodeFilter.SHOW_ELEMENT);
+				while (walker.nextNode()) {
+					// @ts-expect-error It's an Element!
+					contextElement = walker.currentNode;
+				}
+			} else {
+				if (/<body\W/i.test(store.state.contextHTMLForUse)) {
+					contextElement = doc.body;
+				} else if (/<head\W/i.test(store.state.contextHTMLForUse)) {
+					contextElement = doc.head;
+				} else {
+					contextElement = doc.documentElement;
+				}
 			}
 			if (contextElement) {
 				store.state.DOM.contextNode = contextElement.nodeName;
@@ -481,6 +508,7 @@ const store = createStore(NS, {
 		for (const [param, prop] of /** @type {const} */ ([
 			['html', 'html'],
 			['contextHTML', 'contextHTMLForUse'],
+			['selector', 'selector'],
 		])) {
 			if (store.state[prop]) {
 				u.searchParams.set(param, store.state[prop]);
@@ -506,6 +534,7 @@ const store = createStore(NS, {
 				body: JSON.stringify({
 					html: store.state.html,
 					contextHTML: store.state.contextHTMLForUse,
+					selector: store.state.selector,
 				}),
 				headers: {
 					'Content-Type': 'application/json',
@@ -688,6 +717,50 @@ const store = createStore(NS, {
 		const val = /** @type {HTMLInputElement} */ (e.target).valueAsNumber;
 		store.state.playbackPoint = val - 1;
 	},
+
+	/** @param {InputEvent} e */
+	handleSelectorChange: function* (e) {
+		const val = /** @type {HTMLInputElement} */ (e.target).value.trim() || null;
+		if (val) {
+			try {
+				// Test whether the selector is valid before setting it so it isn't applied.
+				document.createDocumentFragment().querySelector(val);
+				store.state.selector = val;
+				store.state.selectorErrorMessage = null;
+				yield store.callAPI();
+				return;
+			} catch (/** @type {unknown} */ e) {
+				if (e instanceof DOMException && e.name === 'SyntaxError') {
+					let msg = e.message;
+
+					/*
+					 * The error message includes methods about our test.
+					 * Chrome:
+					 * > Failed to execute 'querySelector' on 'DocumentFragment': 'foo >' is not a valid selector.
+					 * Firefox:
+					 * > DocumentFragment.querySelector: 'foo >' is not a valid selector
+					 * Safari:
+					 * > 'foo >' is not a valid selector.
+					 *
+					 * Try to strip the irrelevant parts.
+					 */
+					let idx = msg.indexOf(val);
+					if (idx > 0) {
+						if (msg[idx - 1] === '"' || msg[idx - 1] === "'") {
+							idx -= 1;
+						}
+						msg = msg.slice(idx);
+					}
+
+					store.state.selectorErrorMessage = msg;
+				} else {
+					throw e;
+				}
+			}
+		}
+		store.state.selector = null;
+		yield store.callAPI();
+	},
 });
 
 /** @param {keyof State} stateKey */