diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index a388af1ef79c3..46fc192083ec6 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -2218,7 +2218,9 @@ private function class_name_updates_to_attributes_updates() {
* whitespace to a single space, which might appear cleaner
* in the output HTML but produce a noisier change.
*/
- $class .= substr( $existing_class, $ws_at, $ws_length );
+ if ( '' !== $class ) {
+ $class .= substr( $existing_class, $ws_at, $ws_length );
+ }
$class .= $name;
}
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
index b9f696bcfed5c..dd8a849d0b20f 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
@@ -1376,12 +1376,12 @@ public function test_remove_class_removes_a_single_class_from_the_class_attribut
$processor->remove_class( 'main' );
$this->assertSame(
- 'Text
',
+ 'Text
',
$processor->get_updated_html(),
'Updated HTML does not reflect class name removed from existing class attribute via remove_class()'
);
$this->assertSame(
- ' with-border',
+ 'with-border',
$processor->get_attribute( 'class' ),
"get_attribute( 'class' ) does not reflect class name removed from existing class attribute via remove_class()"
);
@@ -1466,12 +1466,12 @@ public function test_add_class_when_there_is_a_class_attribute_with_excessive_wh
$processor->add_class( 'foo-class' );
$this->assertSame(
- 'Text
',
+ 'Text
',
$processor->get_updated_html(),
'Updated HTML does not reflect existing excessive whitespace after adding class name via add_class()'
);
$this->assertSame(
- ' main with-border foo-class',
+ 'main with-border foo-class',
$processor->get_attribute( 'class' ),
"get_attribute( 'class' ) does not reflect existing excessive whitespace after adding class name via add_class()"
);
@@ -1490,12 +1490,12 @@ public function test_remove_class_preserves_whitespaces_when_there_is_a_class_at
$processor->remove_class( 'with-border' );
$this->assertSame(
- 'Text
',
+ 'Text
',
$processor->get_updated_html(),
'Updated HTML does not reflect existing excessive whitespace after removing class name via remove_class()'
);
$this->assertSame(
- ' main',
+ 'main',
$processor->get_attribute( 'class' ),
"get_attribute( 'class' ) does not reflect existing excessive whitespace after removing class name via removing_class()"
);
@@ -1696,8 +1696,8 @@ public function test_advanced_use_case() {
$expected_output = <<