Provide styles for sticky table headers

[scottaohara]

In the ARIA in HTML spec, the document conformance requirements are presented in a rather long table. To help readers of the spec, we modified the style of the table headers so that they could use position: sticky so they'd remain in view while the spec was scrolled.

A recent-ish update to the respec table styling, to better match w3c styles, resulted in the background of the table headers being removed - so now the sticky content has no background color and the header text and the table body text have legibility issues.

Per w3c/html-aria#518, I was told to file an issue here to get official support for this table header treatment, rather than doing it uniquely for the ARIA in HTML spec alone.

In the meantime, I will have to remove the sticky column headers to resolve the legibility issue. But that will of course mean the usability issue of the column headers being visually far removed from the current scroll position will have returned.

[nigelmegitt]

I'd also like to bring back table header background and foreground color adjustment, something a bit like what Respec used to do with table class="simple".