wp-includes/html-api/class-wp-html-open-elements.php:172Returns how many nodes are currently in the stack of open elements.
int public function count(): int { return count( $this->stack ); }Introduced in 6.4.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/html-api/class-wp-html-open-elements.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.