I have a div that is visible, block display, has a height and width, is position:fixed to be in (100,100), and has a z-index higher than everything else on the page.
What possibly could be causing me not to see this div? I can’t post the code itself, sadly, but doesn’t this cover pretty much everything in HTML/CSS that could be causing it not to be displayed?

Other than the properties you mention, there is also
visibilityandopacity. Your screendump doesn’t show inherited styles, so you could check if those are inherited. Also look at any inherited!importantstyles.In addition, the
z-indexonly works correctly if both the element and the parent are positioned. So that could mean thez-indexis ignored here and the div could be obscured by other elements following it later.