The aspects enable a processor to concurrently execute a first serial language code (e.g., HTML) embedding a second serial language code (e.g., JavaScript®) during a page load operation by a browser. A parser parses the first serial language code until a segment of the embedded second serial language code is encountered. The segment of embedded second serial language code is extracted for execution by an execution engine, which proceeds concurrently with speculative parsing of the first serial language code. Code generated by execution of second serial language code is evaluated to determine if it is well formed, and a partial rollback and re-parsing of the first serial language code is performed if the code is not well formed.