This is consistent with other functions such as
HTMLElement::offset_width and fixes a crash for the included test.
Returning an offset of zero is not correct for this case, but this is
still an improvement to not crash.
According to the HTML specification, the `size` attribute of an input
element must be a valid non-negative integer greater than zero. If the
value is invalid or set to `0`, the default size of `20` should be used.
This small change fixes one issue identified in
https://wpt.live/html/rendering/widgets/input-text-size.html
The WPT test suite was also automatically imported.
In the case where we had a preferred aspect ratio and a natural height
but no natural width, we'd get into ping-ponging infinite recursion by
trying to find the width to resolve the height to resolve the width to
resolve the height...
