What is the point of a dedicated tag for a circle when you can just make one easily and more intuitively using border-radius? Inline styling is touted as bad practice, so I don't get the appeal of making a circle using non intuitive attributes inside the DOM.
Here is Optimizely using <circle> for semi-transparent grey circles in the landing page
There are no SO posts I know of, or MDN references helpful enough about the <circle> tag. W3C reference is also unhelpful, hence this post.
There is no <circle> element in HTML. It is an SVG element.
SVG is a vector graphics language, so <circle> is a perfectly reasonable thing for it to contain.
(HTML allows SVG to be embedded in it).