Keep attributes in sync example is dubious

[annevk]

https://w3ctag.github.io/design-principles/#html-idl-must-by-synced

A counterpattern to this guidance can be found in input’s value, option’s selected, and input’s checked where the HTML attributes were never updated and the IDL attribute was the single source of truth.

The problem with these attributes is not that they are not reflected, the problem is that the IDL attributes have a wildly different name. E.g., value is reflected as defaultValue.

[annevk]

In light of #572 this also strikes me as the kind of principle that doesn't really serve a purpose. The HTML community is well aware and has already taken care of this by not repeating that mistake in the last twenty years of HTML's development.