WHY ISN’T THE HTML ELEMENT 100% SUPPORTED ON CANIUSE.COM?

Summary from the TLDR Web Dev newsletter

The html element is not 100% supported on CanIUse because of some deprecated and non-standardized features associated with the element. However, that doesn’t explain it entirely and the exact reasons are unknown, though rounding errors are suspected as a possible cause. CanIUse is a site that lists what HTML tags are supported by browsers.