diff --git a/lib/experimental/html/class-wp-html-tag-processor.php b/lib/experimental/html/class-wp-html-tag-processor.php
index be6179c963571..3f4ff354d6acd 100644
--- a/lib/experimental/html/class-wp-html-tag-processor.php
+++ b/lib/experimental/html/class-wp-html-tag-processor.php
@@ -948,6 +948,38 @@ public function set_attribute( $name, $value ) {
 			return;
 		}
 
+		/*
+		 * Verify that the attribute name is allowable. In WP_DEBUG
+		 * environments we want to crash to quickly alert developers
+		 * of typos and issues; but in production we don't want to
+		 * interrupt a normal page view, so we'll silently avoid
+		 * updating the attribute in those cases.
+		 *
+		 * Of note, we're disallowing more characters than are strictly
+		 * forbidden in HTML5. This is to prevent additional security
+		 * risks deeper in the WordPress and plugin stack. Specifically
+		 * we reject the less-than (<) and ampersand (&) characters.
+		 *
+		 * The use of a PCRE match allows us to look for specific Unicode
+		 * code points without writing a UTF-8 decoder. Whereas scanning
+		 * for one-byte characters is trivial, scanning for the longer
+		 * byte sequences would be more complicated, and this shouldn't
+		 * be in the hot path for execution so we can compromise on the
+		 * efficiency at this point.
+		 *
+		 * @see https://html.spec.whatwg.org/#attributes-2
+		 */
+		if ( preg_match(
+			'~[ "\'>&</=\x{00}-\x{1F}\x{FDD0}-\x{FDEF}\x{FFFE}\x{FFFF}\x{1FFFE}\x{1FFFF}\x{2FFFE}\x{2FFFF}\x{3FFFE}\x{3FFFF}\x{4FFFE}\x{4FFFF}\x{5FFFE}\x{5FFFF}\x{6FFFE}\x{6FFFF}\x{7FFFE}\x{7FFFF}\x{8FFFE}\x{8FFFF}\x{9FFFE}\x{9FFFF}\x{AFFFE}\x{AFFFF}\x{BFFFE}\x{BFFFF}\x{CFFFE}\x{CFFFF}\x{DFFFE}\x{DFFFF}\x{EFFFE}\x{EFFFF}\x{FFFFE}\x{FFFFF}\x{10FFFE}\x{10FFFF}]~Ssu',
+			$name
+		) ) {
+			if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
+				throw new Exception( 'Invalid attribute name' );
+			}
+
+			return;
+		}
+
 		/*
 		 * > The values "true" and "false" are not allowed on boolean attributes.
 		 * > To represent a false value, the attribute has to be omitted altogether.
diff --git a/phpunit/html/wp-html-tag-processor-test.php b/phpunit/html/WP_HTML_Tag_Processor_Test.php
similarity index 95%
rename from phpunit/html/wp-html-tag-processor-test.php
rename to phpunit/html/WP_HTML_Tag_Processor_Test.php
index 41bf04a138abc..eb5badd9fef48 100644
--- a/phpunit/html/wp-html-tag-processor-test.php
+++ b/phpunit/html/WP_HTML_Tag_Processor_Test.php
@@ -248,6 +248,57 @@ public function test_set_attribute_with_a_non_existing_attribute_adds_a_new_attr
 		$this->assertSame( '<div test-attribute="test-value" id="first"><span id="second">Text</span></div>', (string) $p );
 	}
 
+	/**
+	 * Attribute names with invalid characters should be rejected.
+	 *
+	 * > Attributes have a name and a value. Attribute names must
+	 * > consist of one or more characters other than controls,
+	 * > U+0020 SPACE, U+0022 ("), U+0027 ('), U+003E (>),
+	 * > U+002F (/), U+003D (=), and noncharacters.
+	 *
+	 * @see https://html.spec.whatwg.org/#attributes-2
+	 *
+	 * @dataProvider data_invalid_attribute_names
+	 * @covers set_attribute
+	 */
+	public function test_set_attribute_rejects_invalid_attribute_names( $attribute_name ) {
+		$p = new WP_HTML_Tag_Processor( '<span></span>' );
+
+		$this->expectException( Exception::class );
+
+		$p->next_tag();
+		$p->set_attribute( $attribute_name, "test" );
+
+		$this->assertEquals( '<span></span>', (string) $p );
+	}
+
+	/**
+	 * Data provider with invalid HTML attribute names.
+	 *
+	 * @return array {
+	 *     @type string $attribute_name Text considered invalid for HTML attribute names.
+	 * }
+	 */
+	public function data_invalid_attribute_names() {
+		return array(
+			'controls_null'    => array( "i\x00d" ),
+			'controls_newline' => array( "\nbroken-expectations" ),
+			'space'            => array( "aria label" ),
+			'double-quote'     => array( '"id"' ),
+			'single-quote'     => array( "'id'" ),
+			'greater-than'     => array( 'sneaky>script' ),
+			'solidus'          => array( 'data/test-id' ),
+			'equals'           => array( 'checked=checked' ),
+			'noncharacters_1'  => array( html_entity_decode( 'anything&#xFDD0;' ) ),
+			'noncharacters_2'  => array( html_entity_decode( 'te&#xFFFF;st' ) ),
+			'noncharacters_3'  => array( html_entity_decode( 'te&#x2FFFE;st' ) ),
+			'noncharacters_4'  => array( html_entity_decode( 'te&#xDFFFF;st' ) ),
+			'noncharacters_5'  => array( html_entity_decode( '&#x10FFFE;' ) ),
+			'wp_no_lt'         => array( 'id<script'),
+			'wp_no_amp'        => array( 'class&lt;script'),
+		);
+	}
+
 	/**
 	 * According to HTML spec, only the first instance of an attribute counts.
 	 * The other ones are ignored.