Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 45928)
+++ src/wp-includes/formatting.php	(working copy)
@@ -2429,7 +2429,7 @@
  * @return string Balanced text
  */
 function balanceTags( $text, $force = false ) {  // phpcs:ignore WordPress.NamingConventions.ValidFunctionName.FunctionNameInvalid
-	if ( $force || get_option( 'use_balanceTags' ) == 1 ) {
+	if ( $force || (int) get_option( 'use_balanceTags' ) === 1 ) {
 		return force_balance_tags( $text );
 	} else {
 		return $text;
@@ -2440,6 +2440,7 @@
  * Balances tags of string using a modified stack.
  *
  * @since 2.0.4
+ * @since 5.3.0 Improve accuracy and add support for custom element tags.
  *
  * @author Leonard Lin <leonard@acm.org>
  * @license GPL
@@ -2469,32 +2470,76 @@
 	// WP bug fix for LOVE <3 (and other situations with '<' before a number)
 	$text = preg_replace( '#<([0-9]{1})#', '&lt;$1', $text );
 
-	while ( preg_match( '/<(\/?[\w:]*)\s*([^>]*)>/', $text, $regex ) ) {
+	/**
+	 * Matches supported tags
+	 *
+	 * To get the pattern as a string without the comments paste into a PHP
+	 * REPL like `php -a`
+	 *
+	 * @see https://html.spec.whatwg.org/#elements-2
+	 * @see https://w3c.github.io/webcomponents/spec/custom/#valid-custom-element-name
+	 *
+	 * @example
+	 * ~# php -a
+	 * php > $s = [paste copied contents of expression below including parentheses];
+	 * php > echo $s;
+	 *
+	 * @type string
+	 */
+	$tag_pattern = (
+		'#<' . // Start with an opening bracket.
+		'(/?)' . // Group 1 - If it's a closing tag it'll have a leading slash.
+		'(' . // Group 2 - Tag name.
+			// Custom element tags have more lenient rules than HTML tag names.
+			'(?:[a-z](?:[a-z0-9._]*)-(?:[a-z0-9._-]+)+)' .
+				'|' .
+			// Traditional tag rules approximate HTML tag names.
+			'(?:[\w:]+)' .
+		')' .
+		'(?:' .
+			// We either immediately close the tag with its '>' and have nothing here.
+			'\s*' .
+			'(/?)' . // Group 3 - "attributes" for empty tag.
+				'|' .
+			// Or we must start with space characters to separate the tag name from the attributes (or whitespace).
+			'(\s+)' . // Group 4 - Pre-attribute whitespace.
+			'([^>]*)' . // Group 5 - Attributes.
+		')' .
+		'>#' // End with a closing bracket.
+	);
+
+	while ( preg_match( $tag_pattern, $text, $regex ) ) {
+		$full_match        = $regex[0];
+		$has_leading_slash = ! empty( $regex[1] );
+		$tag_name          = $regex[2];
+		$tag               = strtolower( $tag_name );
+		$is_single_tag     = in_array( $tag, $single_tags, true );
+		$pre_attribute_ws  = isset( $regex[4] ) ? $regex[4] : '';
+		$attributes        = trim( isset( $regex[5] ) ? $regex[5] : $regex[3] );
+		$has_self_closer   = '/' === substr( $attributes, -1 );
+
 		$newtext .= $tagqueue;
 
-		$i = strpos( $text, $regex[0] );
-		$l = strlen( $regex[0] );
+		$i = strpos( $text, $full_match );
+		$l = strlen( $full_match );
 
-		// clear the shifter
+		// Clear the shifter.
 		$tagqueue = '';
-		// Pop or Push
-		if ( isset( $regex[1][0] ) && '/' == $regex[1][0] ) { // End Tag
-			$tag = strtolower( substr( $regex[1], 1 ) );
-			// if too many closing tags
+		if ( $has_leading_slash ) { // End Tag.
+			// If too many closing tags.
 			if ( $stacksize <= 0 ) {
 				$tag = '';
-				// or close to be safe $tag = '/' . $tag;
+				// Or close to be safe $tag = '/' . $tag.
 
-				// if stacktop value = tag close value then pop
-			} elseif ( $tagstack[ $stacksize - 1 ] == $tag ) { // found closing tag
-				$tag = '</' . $tag . '>'; // Close Tag
-				// Pop
+				// If stacktop value = tag close value, then pop.
+			} elseif ( $tagstack[ $stacksize - 1 ] === $tag ) { // Found closing tag.
+				$tag = '</' . $tag . '>'; // Close Tag.
 				array_pop( $tagstack );
 				$stacksize--;
-			} else { // closing tag not at top, search for it
+			} else { // Closing tag not at top, search for it.
 				for ( $j = $stacksize - 1; $j >= 0; $j-- ) {
-					if ( $tagstack[ $j ] == $tag ) {
-						// add tag to tagqueue
+					if ( $tagstack[ $j ] === $tag ) {
+						// Add tag to tagqueue.
 						for ( $k = $stacksize - 1; $k >= $j; $k-- ) {
 							$tagqueue .= '</' . array_pop( $tagstack ) . '>';
 							$stacksize--;
@@ -2504,39 +2549,33 @@
 				}
 				$tag = '';
 			}
-		} else { // Begin Tag
-			$tag = strtolower( $regex[1] );
-
-			// Tag Cleaning
-
-			// If it's an empty tag "< >", do nothing
-			if ( '' == $tag ) {
-				// do nothing
-			} elseif ( substr( $regex[2], -1 ) == '/' ) { // ElseIf it presents itself as a self-closing tag...
+		} else { // Begin Tag.
+			if ( $has_self_closer ) { // If it presents itself as a self-closing tag...
 				// ...but it isn't a known single-entity self-closing tag, then don't let it be treated as such and
 				// immediately close it with a closing tag (the tag will encapsulate no text as a result)
-				if ( ! in_array( $tag, $single_tags ) ) {
-					$regex[2] = trim( substr( $regex[2], 0, -1 ) ) . "></$tag";
+				if ( ! $is_single_tag ) {
+					$attributes = trim( substr( $attributes, 0, -1 ) ) . "></$tag";
 				}
-			} elseif ( in_array( $tag, $single_tags ) ) { // ElseIf it's a known single-entity tag but it doesn't close itself, do so
-				$regex[2] .= '/';
-			} else { // Else it's not a single-entity tag
-				// If the top of the stack is the same as the tag we want to push, close previous tag
-				if ( $stacksize > 0 && ! in_array( $tag, $nestable_tags ) && $tagstack[ $stacksize - 1 ] == $tag ) {
+			} elseif ( $is_single_tag ) { // ElseIf it's a known single-entity tag but it doesn't close itself, do so
+				$pre_attribute_ws = ' ';
+				$attributes      .= '/';
+			} else { // It's not a single-entity tag.
+				// If the top of the stack is the same as the tag we want to push, close previous tag.
+				if ( $stacksize > 0 && ! in_array( $tag, $nestable_tags, true ) && $tagstack[ $stacksize - 1 ] === $tag ) {
 					$tagqueue = '</' . array_pop( $tagstack ) . '>';
 					$stacksize--;
 				}
 				$stacksize = array_push( $tagstack, $tag );
 			}
 
-			// Attributes
-			$attributes = $regex[2];
-			if ( ! empty( $attributes ) && $attributes[0] != '>' ) {
-				$attributes = ' ' . $attributes;
+			// Attributes.
+			if ( $has_self_closer && $is_single_tag ) {
+				// We need some space - avoid <br/> and prefer <br />.
+				$pre_attribute_ws = ' ';
 			}
 
-			$tag = '<' . $tag . $attributes . '>';
-			//If already queuing a close tag, then put this tag on, too
+			$tag = '<' . $tag . $pre_attribute_ws . $attributes . '>';
+			// If already queuing a close tag, then put this tag on too.
 			if ( ! empty( $tagqueue ) ) {
 				$tagqueue .= $tag;
 				$tag       = '';
@@ -2546,18 +2585,17 @@
 		$text     = substr( $text, $i + $l );
 	}
 
-	// Clear Tag Queue
+	// Clear Tag Queue.
 	$newtext .= $tagqueue;
 
-	// Add Remaining text
+	// Add remaining text.
 	$newtext .= $text;
 
-	// Empty Stack
 	while ( $x = array_pop( $tagstack ) ) {
-		$newtext .= '</' . $x . '>'; // Add remaining tags to close
+		$newtext .= '</' . $x . '>'; // Add remaining tags to close.
 	}
 
-	// WP fix for the bug with HTML comments
+	// WP fix for the bug with HTML comments.
 	$newtext = str_replace( '< !--', '<!--', $newtext );
 	$newtext = str_replace( '<    !--', '< !--', $newtext );
 
Index: tests/phpunit/tests/formatting/balanceTags.php
===================================================================
--- tests/phpunit/tests/formatting/balanceTags.php	(revision 45928)
+++ tests/phpunit/tests/formatting/balanceTags.php	(working copy)
@@ -37,6 +37,158 @@
 		);
 	}
 
+	function supported_traditional_tag_names() {
+		return array(
+			array( 'a' ),
+			array( 'div' ),
+			array( 'blockquote' ),
+			// HTML tag names can be CAPITALIZED and are case-insensitive.
+			array( 'A' ),
+			array( 'dIv' ),
+			array( 'BLOCKQUOTE' ),
+		);
+	}
+
+	function supported_custom_element_tag_names() {
+		return array(
+			array( 'custom-element' ),
+			array( 'my-custom-element' ),
+			array( 'weekday-5-item' ),
+			array( 'a-big-old-tag-name' ),
+			array( 'with_underscores-and_the_dash' ),
+			array( 'a-.' ),
+			array( 'a._-.-_' ),
+		);
+	}
+
+	function invalid_tag_names() {
+		return array(
+			array( '<0-day>inside', '&lt;0-day>inside' ), // Can't start with a number - handled by the "<3" fix.
+			array( '<UPPERCASE-TAG>inside', '<UPPERCASE-TAG>inside' ), // Custom elements cannot be uppercase.
+		);
+	}
+
+	/**
+	 * These are valid custom elements but we don't support them yet.
+	 *
+	 * @see https://w3c.github.io/webcomponents/spec/custom/#valid-custom-element-name
+	 */
+	function unsupported_valid_tag_names() {
+		return array(
+			// We don't allow ending in a dash.
+			array( '<what->inside' ),
+			// Examples from the spec working document.
+			array( 'math-α' ),
+			array( 'emotion-😍' ),
+			// UNICODE ranges
+			// 0x00b7
+			array( 'b-·' ),
+			// Latin characters with accents/modifiers.
+			// 0x00c0-0x00d6
+			// 0x00d8-0x00f6
+			array( 'a-À-Ó-Ý' ),
+			// 0x00f8-0x037d
+			array( 'a-ͳ' ),
+			// No 0x037e, which is a Greek semicolon.
+			// 0x037f-0x1fff
+			array( 'a-Ფ' ),
+			// Zero-width characters, probably never supported.
+			// 0x200c-0x200d
+			array( 'a-‌to-my-left-is-a-zero-width-non-joiner-do-not-delete-it' ),
+			array( 'a-‍to-my-left-is-a-zero-width-joiner-do-not-delete-it' ),
+			// Ties.
+			// 0x203f-0x2040
+			array( 'under-‿-tie' ),
+			array( 'over-⁀-tie' ),
+			// 0x2170-0x218f
+			array( 'a-⁰' ),
+			array( 'a-⅀' ),
+			array( 'tag-ↀ-it' ),
+			// 0x2c00-0x2fef
+			array( 'a-Ⰰ' ),
+			array( 'b-ⴓ-c' ),
+			array( 'd-⽗' ),
+			// 0x3001-0xd7ff
+			array( 'a-、' ),
+			array( 'z-态' ),
+			array( 'a-送-䠺-ퟱ-퟿' ),
+			// 0xf900-0xfdcf
+			array( 'a-豈' ),
+			array( 'my-切' ),
+			array( 'aﴀ-tag' ),
+			array( 'my-﷌' ),
+			// 0xfdf0-0xfffd
+			array( 'a-ﷰ' ),
+			array( 'a-￰-￸-�' ), // Warning; blank characters are in there.
+			// Extended ranges.
+			// 0x10000-0xeffff
+			array( 'a-𐀀' ),
+			array( 'my-𝀀' ),
+			array( 'a𞀀-𜿐' ),
+		);
+	}
+
+	/**
+	 * These are invalid custom elements but we support them right now in order to keep the parser simpler.
+	 *
+	 * @see https://w3c.github.io/webcomponents/spec/custom/#valid-custom-element-name
+	 */
+	function supported_invalid_tag_names() {
+		return array(
+			// Reserved names for custom elements.
+			array( 'annotation-xml' ),
+			array( 'color-profile' ),
+			array( 'font-face' ),
+			array( 'font-face-src' ),
+			array( 'font-face-uri' ),
+			array( 'font-face-format' ),
+			array( 'font-face-name' ),
+			array( 'missing-glyph' ),
+		);
+	}
+
+	/**
+	 * @ticket 47014
+	 * @dataProvider supported_traditional_tag_names
+	 */
+	function test_detects_traditional_tag_names( $tag ) {
+		$normalized = strtolower( $tag );
+
+		$this->assertEquals( "<$normalized>inside</$normalized>", balanceTags( "<$tag>inside", true ) );
+	}
+
+	/**
+	 * @ticket 47014
+	 * @dataProvider supported_custom_element_tag_names
+	 */
+	function test_detects_supported_custom_element_tag_names( $tag ) {
+		$this->assertEquals( "<$tag>inside</$tag>", balanceTags( "<$tag>inside", true ) );
+	}
+
+	/**
+	 * @ticket 47014
+	 * @dataProvider invalid_tag_names
+	 */
+	function test_ignores_invalid_tag_names( $input, $output ) {
+		$this->assertEquals( $output, balanceTags( $input, true ) );
+	}
+
+	/**
+	 * @ticket 47014
+	 * @dataProvider unsupported_valid_tag_names
+	 */
+	function test_ignores_unsupported_custom_tag_names( $tag ) {
+		$this->assertEquals( "<$tag>inside", balanceTags( "<$tag>inside", true ) );
+	}
+
+	/**
+	 * @ticket 47014
+	 * @dataProvider supported_invalid_tag_names
+	 */
+	function test_detects_supported_invalid_tag_names( $tag ) {
+		$this->assertEquals( "<$tag>inside</$tag>", balanceTags( "<$tag>inside", true ) );
+	}
+
 	/**
 	 * If a recognized valid single tag appears unclosed, it should get self-closed
 	 *
@@ -68,12 +220,15 @@
 			'<em />',
 			'<p class="main1"/>',
 			'<p class="main2" />',
+			'<STRONG/>',
 		);
 		$expected = array(
 			'<strong></strong>',
 			'<em></em>',
 			'<p class="main1"></p>',
 			'<p class="main2"></p>',
+			// Valid tags are transformed to lowercase.
+			'<strong></strong>',
 		);
 
 		foreach ( $inputs as $key => $input ) {
@@ -221,4 +376,68 @@
 		}
 	}
 
+	/**
+	 * Get custom element data.
+	 *
+	 * @return array Data.
+	 */
+	public function data_custom_elements() {
+		return array(
+			// Valid custom element tags.
+			array(
+				'<my-custom-element data-attribute="value"/>',
+				'<my-custom-element data-attribute="value"></my-custom-element>',
+			),
+			array(
+				'<my-custom-element>Test</my-custom-element>',
+				'<my-custom-element>Test</my-custom-element>',
+			),
+			array(
+				'<my-custom-element>Test',
+				'<my-custom-element>Test</my-custom-element>',
+			),
+			array(
+				'Test</my-custom-element>',
+				'Test',
+			),
+			array(
+				'</my-custom-element>Test',
+				'Test',
+			),
+			array(
+				'<my-custom-element/>',
+				'<my-custom-element></my-custom-element>',
+			),
+			array(
+				'<my-custom-element />',
+				'<my-custom-element></my-custom-element>',
+			),
+			// Invalid (or at least temporarily unsupported) custom element tags.
+			array(
+				'<MY-CUSTOM-ELEMENT>Test',
+				'<MY-CUSTOM-ELEMENT>Test',
+			),
+			array(
+				'<my->Test',
+				'<my->Test',
+			),
+			array(
+				'<--->Test',
+				'<--->Test',
+			),
+		);
+	}
+
+	/**
+	 * Test custom elements.
+	 *
+	 * @ticket 47014
+	 * @dataProvider data_custom_elements
+	 *
+	 * @param string $source   Source.
+	 * @param string $expected Expected.
+	 */
+	public function test_custom_elements( $source, $expected ) {
+		$this->assertEquals( $expected, balanceTags( $source, true ) );
+	}
 }