No tags are recognized in a
title element by HTML rules (and by browser practice); they are indeed rendered as such. In principle, you can use special characters like “₂” SUBSCRIPT TWO U+2082 in a
title element. You can even denote them using character references:
However, only some fonts support SUBSCRIPT TWO, and it is quite possible that the rendering of
title elements e.g. in a browser top bar or tab label uses a font that does not support to it. In that case, a rectangle, vertical bar, space, or other generic indicator of missing glyph probably appears.