Due to implementation details, the HTML5 facet of Validator.nu used to ignore the content of obsolete elements such as
center, because obsolete elements were simply unknown. This wasn’t particularly useful when assessing the HTML5-upgradeability of an existing design that wrapped everything in
center, for example.
The HTML5 facet of Validator.nu now knows about obsolete container elements that existed as deprecated in HTML 4.01. This means that
center is still an error, but the contents are now checked as HTML5.
Also, Validator.nu now allows legacy-style internal encoding declarations per the latest Editor’s Draft.