Index: src/wp-includes/class-wp-embed.php
===================================================================
--- src/wp-includes/class-wp-embed.php	(revision 33407)
+++ src/wp-includes/class-wp-embed.php	(working copy)
@@ -318,11 +318,20 @@
 	 * @return string Potentially modified $content.
 	 */
 	public function autoembed( $content ) {
-		// Strip newlines from all elements.
-		$content = wp_replace_in_html_tags( $content, array( "\n" => " " ) );
+		// Avoid multiline elements and comments.
+		$content = wp_html_split( $content );
+		for ( $i = 0, $c = count( $content ); $i < $c; $i += 2 ) { 
 
-		// Find URLs that are on their own line.
-		return preg_replace_callback( '|^(\s*)(https?://[^\s"]+)(\s*)$|im', array( $this, 'autoembed_callback' ), $content );
+			if ( '' == $content[$i] || false === strpos( $content[$i], 'http' ) ) {
+				continue;
+			}
+
+			// Find URLs that are on their own line.
+			$content[$i] = preg_replace_callback( '|^(\s*)(https?://[^\s"]+)(\s*)$|im', array( $this, 'autoembed_callback' ), $content[$i] );
+
+		}
+
+		return implode( $content );
 	}
 
 	/**
Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 33407)
+++ src/wp-includes/formatting.php	(working copy)
@@ -504,8 +504,8 @@
 	// Standardize newline characters to "\n".
 	$pee = str_replace(array("\r\n", "\r"), "\n", $pee);
 
-	// Strip newlines from all elements.
-	$pee = wp_replace_in_html_tags( $pee, array( "\n" => " " ) );
+	// Find newlines in all elements and add placeholders.
+	$pee = wp_replace_in_html_tags( $pee, array( "\n" => " <!-- wpnl --> " ) );
 
 	// Collapse line breaks before and after <option> elements so they don't get autop'd.
 	if ( strpos( $pee, '<option' ) !== false ) {
@@ -592,19 +592,21 @@
 	if ( !empty($pre_tags) )
 		$pee = str_replace(array_keys($pre_tags), array_values($pre_tags), $pee);
 
+	// Restore newlines in all elements.
+	$pee = str_replace( " <!-- wpnl --> ", "\n", $pee );
+
 	return $pee;
 }
 
 /**
- * Replace characters or phrases within HTML elements only.
+ * Separate HTML elements and comments from the text.
  *
- * @since 4.2.3
+ * @since 4.2.4
  *
- * @param string $haystack The text which has to be formatted.
- * @param array $replace_pairs In the form array('from' => 'to', ...).
- * @return string The formatted text.
+ * @param string $input The text which has to be formatted.
+ * @return array The formatted text.
  */
-function wp_replace_in_html_tags( $haystack, $replace_pairs ) {
+function wp_html_split( $input ) {
 	// Find all elements.
 	$comments =
 		  '!'           // Start of comment, after the <.
@@ -614,6 +616,15 @@
 		. ')*+'         // Loop possessively.
 		. '(?:-->)?';   // End of comment. If not found, match all input.
 
+	$cdata =
+		  '!\[CDATA\['  // Start of comment, after the <.
+		. '[^\]]*+'     // Consume non-].
+		. '(?:'         // Unroll the loop: Consume everything until ]]> is found.
+		.     '](?!]>)' // One ] not followed by end of comment.
+		.     '[^\]]*+' // Consume non-].
+		. ')*+'         // Loop possessively.
+		. '(?:]]>)?';   // End of comment. If not found, match all input.
+
 	$regex =
 		  '/('              // Capture the entire match.
 		.     '<'           // Find start of element.
@@ -620,11 +631,29 @@
 		.     '(?(?=!--)'   // Is this a comment?
 		.         $comments // Find end of comment.
 		.     '|'
-		.         '[^>]*>?' // Find end of element. If not found, match all input.
+		.         '(?(?=!\[CDATA\[)' // Is this a comment?
+		.             $cdata // Find end of comment.
+		.         '|'
+		.             '[^>]*>?' // Find end of element. If not found, match all input.
+		.         ')'
 		.     ')'
 		. ')/s';
 
-	$textarr = preg_split( $regex, $haystack, -1, PREG_SPLIT_DELIM_CAPTURE );
+	return preg_split( $regex, $input, -1, PREG_SPLIT_DELIM_CAPTURE );
+}
+
+/**
+ * Replace characters or phrases within HTML elements only.
+ *
+ * @since 4.2.3
+ *
+ * @param string $haystack The text which has to be formatted.
+ * @param array $replace_pairs In the form array('from' => 'to', ...).
+ * @return string The formatted text.
+ */
+function wp_replace_in_html_tags( $haystack, $replace_pairs ) {
+	// Find all elements.
+	$textarr = wp_html_split( $haystack );
 	$changed = false;
 
 	// Optimize when searching for one item.
Index: tests/phpunit/tests/formatting/Autop.php
===================================================================
--- tests/phpunit/tests/formatting/Autop.php	(revision 33407)
+++ tests/phpunit/tests/formatting/Autop.php	(working copy)
@@ -399,4 +399,50 @@
 
 		$this->assertEquals( $expected, trim( wpautop( $content ) ) );
 	}
+
+	/**
+	 * Do not allow newlines within HTML elements to become mangled.
+	 *
+	 * @ticket 33106
+	 * @dataProvider data_element_sanity
+	 */
+	function test_element_sanity( $input, $output ) {
+		return $this->assertEquals( $output, wpautop( $input ) );
+	}
+
+	function data_element_sanity() {
+		return array(
+			array(
+				"Hello <a\nhref='world'>",
+				"<p>Hello <a\nhref='world'></p>\n",
+			),
+			array(
+				"Hello <!-- a\nhref='world' -->",
+				"<p>Hello <!-- a\nhref='world' --></p>\n",
+			),
+/* Block elements inside comments will fail this test in all versions, it's not a regression.
+			array(
+				"Hello <!-- <hr> a\nhref='world' -->",
+				"<p>Hello <!-- <hr> a\nhref='world' --></p>\n",
+			),
+			array(
+				"Hello <![CDATA[ <hr> a\nhttps://youtu.be/jgz0uSaOZbE\n ]]>",
+				"<p>Hello <![CDATA[ <hr> a\nhttps://youtu.be/jgz0uSaOZbE\n ]]></p>\n",
+			),
+*/
+			array(
+				"Hello <![CDATA[ a\nhttps://youtu.be/jgz0uSaOZbE\n ]]>",
+				"<p>Hello <![CDATA[ a\nhttps://youtu.be/jgz0uSaOZbE\n ]]></p>\n",
+			),
+			array(
+				"Hello <![CDATA[ <!-- a\nhttps://youtu.be/jgz0uSaOZbE\n a\n9 ]]> -->",
+				"<p>Hello <![CDATA[ <!-- a\nhttps://youtu.be/jgz0uSaOZbE\n a\n9 ]]> --></p>\n",
+			),
+			array(
+				"Hello <![CDATA[ <!-- a\nhttps://youtu.be/jgz0uSaOZbE\n a\n9 --> a\n9 ]]>",
+				"<p>Hello <![CDATA[ <!-- a\nhttps://youtu.be/jgz0uSaOZbE\n a\n9 --> a\n9 ]]></p>\n",
+			),
+		);
+	}
+	
 }