This porting effort makes it pretty clear we will want a UTF-16-aware
GenericLexer. But for now, we can actually make ASCII assumptions about
what we are parsing, and act accordingly.
This change — part of the HTML constraint-validation API (aka
“client-side form validation”) — implements the willValidate IDL/DOM
attribute/property for all form controls that support it.
Previously, the`HTMLInputElement.selectinStart` and
`HTMLInputElement.selectionEnd` IDL setters, and the
`setRangeText()` IDL method were used when updating an input's value
on keyboard input. These methods can't be used for this purpose,
since selection doesn't apply to email type inputs. Therefore, this
change introduces internal-use only methods that don't check whether
selection applies to the given input.
