HTML Checking for Large Sites
Rocket Validator integrates the W3C Validator HTML checker into an automated web crawler.
HTML issues tagged as unallowed attribute.
The seamless attribute was proposed to be included in the HTML5 spec, but it wasn’t finally accepted, so it’s not a valid attribute for <iframe>.
When was the last time you validated your whole site?
Keep your sites healthy checking for A11Y/HTML issues on an automated schedule.