diff --git a/wp-includes/class-wp-token-map.php b/wp-includes/class-wp-token-map.php
index 09a0b9303b..fc223b187f 100644
--- a/wp-includes/class-wp-token-map.php
+++ b/wp-includes/class-wp-token-map.php
@@ -440,6 +440,10 @@ class WP_Token_Map {
 	 * @return bool Whether there's an entry for the given word in the map.
 	 */
 	public function contains( string $word, string $case_sensitivity = 'case-sensitive' ): bool {
+		if ( str_contains( $word, "\x00" ) ) {
+			return false;
+		}
+
 		$ignore_case = 'ascii-case-insensitive' === $case_sensitivity;
 
 		if ( $this->key_length >= strlen( $word ) ) {
@@ -533,9 +537,17 @@ class WP_Token_Map {
 
 		// Search for a long word first, if the text is long enough, and if that fails, a short one.
 		if ( $text_length > $this->key_length ) {
-			$group_key = substr( $text, $offset, $this->key_length );
+			/*
+			 * Keys cannot contain null bytes, which is taken care of for the full words,
+			 * but here it’s required to reject group keys with null bytes so that the
+			 * lookup doesn’t get off track when scanning the group string.
+			 */
+			if ( strcspn( $text, "\x00", $offset, $this->key_length ) < $this->key_length ) {
+				return null;
+			}
 
-			$group_at = $ignore_case ? stripos( $this->groups, $group_key ) : strpos( $this->groups, $group_key );
+			$group_key = substr( $text, $offset, $this->key_length );
+			$group_at  = $ignore_case ? stripos( $this->groups, $group_key ) : strpos( $this->groups, $group_key );
 			if ( false === $group_at ) {
 				// Perhaps a short word then.
 				return strlen( $this->small_words ) > 0
diff --git a/wp-includes/html-api/class-wp-html-open-elements.php b/wp-includes/html-api/class-wp-html-open-elements.php
index e17f901c4d..0cd1f0fc45 100644
--- a/wp-includes/html-api/class-wp-html-open-elements.php
+++ b/wp-includes/html-api/class-wp-html-open-elements.php
@@ -738,7 +738,11 @@ class WP_HTML_Open_Elements {
 		 * When adding support for new elements, expand this switch to trap
 		 * cases where the precalculated value needs to change.
 		 */
-		switch ( $item->node_name ) {
+		$namespaced_name = 'html' === $item->namespace
+			? $item->node_name
+			: "{$item->namespace} {$item->node_name}";
+
+		switch ( $namespaced_name ) {
 			case 'APPLET':
 			case 'BUTTON':
 			case 'CAPTION':
diff --git a/wp-includes/html-api/class-wp-html-processor.php b/wp-includes/html-api/class-wp-html-processor.php
index d9d0d365c6..35d91fad31 100644
--- a/wp-includes/html-api/class-wp-html-processor.php
+++ b/wp-includes/html-api/class-wp-html-processor.php
@@ -813,8 +813,14 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 		 *       until there are events or until there are no more
 		 *       tokens works in the meantime and isn't obviously wrong.
 		 */
-		if ( empty( $this->element_queue ) && $this->step() ) {
-			return $this->next_visitable_token();
+		if ( empty( $this->element_queue ) ) {
+			if ( $this->step() ) {
+				return $this->next_visitable_token();
+			}
+
+			if ( isset( $this->last_error ) ) {
+				return false;
+			}
 		}
 
 		// Process the next event on the queue.
@@ -1401,6 +1407,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 		$tag_name       = str_replace( "\x00", "\u{FFFD}", $this->get_tag() );
 		$in_html        = 'html' === $this->get_namespace();
 		$qualified_name = $in_html ? strtolower( $tag_name ) : $this->get_qualified_tag_name();
+		$qualified_name = str_replace( "\x00", "\u{FFFD}", $qualified_name );
 
 		if ( $this->is_tag_closer() ) {
 			$html .= "</{$qualified_name}>";
@@ -1414,15 +1421,36 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 		}
 
 		$html .= "<{$qualified_name}";
+
+		$previous_attribute_was_true = false;
+		$seen_attribute_names        = array();
 		foreach ( $attribute_names as $attribute_name ) {
-			$html .= " {$this->get_qualified_attribute_name( $attribute_name )}";
+			$qualified_attribute_name = $this->get_qualified_attribute_name( $attribute_name );
+			$qualified_attribute_name = str_replace( "\x00", "\u{FFFD}", $qualified_attribute_name );
+			$qualified_attribute_name = wp_scrub_utf8( $qualified_attribute_name );
+			if ( isset( $seen_attribute_names[ $qualified_attribute_name ] ) ) {
+				continue;
+			} else {
+				$seen_attribute_names[ $qualified_attribute_name ] = true;
+			}
+
+			if (
+				$previous_attribute_was_true &&
+				isset( $qualified_attribute_name[0] ) &&
+				'=' === $qualified_attribute_name[0]
+			) {
+				$html .= '=""';
+			}
+
+			$html .= " {$qualified_attribute_name}";
 			$value = $this->get_attribute( $attribute_name );
 
 			if ( is_string( $value ) ) {
 				$html .= '="' . htmlspecialchars( $value, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5 ) . '"';
 			}
 
-			$html = str_replace( "\x00", "\u{FFFD}", $html );
+			$previous_attribute_was_true = true === $value;
+			$html                        = str_replace( "\x00", "\u{FFFD}", $html );
 		}
 
 		if ( ! $in_html && $this->has_self_closing_flag() ) {
@@ -2667,8 +2695,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 			 */
 			case '-FORM':
 				if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
-					$node                      = $this->state->form_element;
-					$this->state->form_element = null;
+					$node = $this->state->form_element;
 
 					/*
 					 * > If node is null or if the stack of open elements does not have node
@@ -2681,10 +2708,20 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 						null === $node ||
 						! $this->state->stack_of_open_elements->has_element_in_scope( 'FORM' )
 					) {
-						// Parse error: ignore the token.
+						/*
+						 * Parse error: ignore the token.
+						 *
+						 * Keep the form pointer intact when the end tag is ignored, such as
+						 * when a FORM closing tag appears inside an SVG TITLE integration
+						 * point. Otherwise the ignored token changes parser state in a way
+						 * that serialization cannot represent, allowing a later FORM opener
+						 * to appear in the first normalization pass and disappear on the second.
+						 */
 						return $this->step();
 					}
 
+					$this->state->form_element = null;
+
 					$this->generate_implied_end_tags();
 					if ( $node !== $this->state->stack_of_open_elements->current_node() ) {
 						// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
diff --git a/wp-includes/html-api/class-wp-html-tag-processor.php b/wp-includes/html-api/class-wp-html-tag-processor.php
index 4015b352c1..77c1a471db 100644
--- a/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -1424,7 +1424,7 @@ class WP_HTML_Tag_Processor {
 			$this->tag_name_starts_at = $at;
 
 			// Fail if there is no possible tag closer.
-			if ( false === $at || ( $at + $tag_length ) >= $doc_length ) {
+			if ( false === $at || ( $at + 2 + $tag_length ) >= $doc_length ) {
 				return false;
 			}
 
@@ -1815,6 +1815,12 @@ class WP_HTML_Tag_Processor {
 
 					// Abruptly-closed empty comments are a sequence of dashes followed by `>`.
 					$span_of_dashes = strspn( $html, '-', $closer_at );
+					if ( $doc_length <= $span_of_dashes + $closer_at ) {
+						$this->parser_state = self::STATE_INCOMPLETE_INPUT;
+
+						return false;
+					}
+
 					if ( '>' === $html[ $closer_at + $span_of_dashes ] ) {
 						/*
 						 * @todo When implementing `set_modifiable_text()` ensure that updates to this token
diff --git a/wp-includes/version.php b/wp-includes/version.php
index de84974c14..1261e7cc66 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.1-alpha-62438';
+$wp_version = '7.1-alpha-62439';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.