Implement non-zero dimension attribute parsing

Fixes #8445

The only attributes I found that we have implemented that uses non-zero
dimenion attributes:

* `width` for `<td>` and `<th>` (table cells)
* `width` for `<table>`

I updated these implementations to use the new non-zero dimension
attribute parsing and added associated regression tests.

tests/wpt/web-platform-tests/html/rendering/non-replaced-elements/tables/table-width-ref.html

@@ -0,0 +1,13 @@
+<style>
+p {
+  padding: 0;
+  margin: 0;
+}
+</style>
+
+<p>a b</p>
+
+<hr>
+
+<p>a</p>
+<p>b</p>