CSS: Make the reliableTrDimensions support test work with Bootstrap CSS (3.x version)

CSS: Make the reliableTrDimensions support test work with Bootstrap CSS (3.x version)

Bootstrap 5 includes the following CSS on the page:

```css
*,
*::before,
*::after {
  box-sizing: border-box;
}
```

That threw our `reliableTrDimensions` support test off. This change fixes the
support test and adds a unit test ensuring support test values on a page
including Bootstrap 5 CSS are the same as on a page without it.

Fixes gh-5270
Closes gh-5279
Ref gh-5278