HTML
How often I use
I use HTML on a daily basis, often as Razor-flavored HTML in various C#/.NET MVC applications. My workplace's website is a mix of static HTML, .NET applications with Razor, and Cold Fusion pages, all of which use HTML as the base presentation layer.
My skills with HTML
I can and do write semantic, accessible HTML by properly using landmarks, ARIA attributes, and structure. I keep my styling in CSS files. I do NOT use headings simply to make text larger. I'm sure I annoy some of my team with my HTML nitpicks, but I firmly believe a good presentation layer starts with good, sensible structure.
I very rarely need to look up parts of the HTML specification since I used it so frequently, but there are various elements I don't work with at all: the audio and video elements in HTML are something I haven't had much occasion to use, for instance. I'd have to look those up.