Pure HTML for older browsers. The following HTML source
√<span style="text-decoration: overline">2</span>is displayed as
√2Not all browsers display the above equally well, but it is always readable.
MathML for newer browsers. The following HTML source
<math><msqrt><mi>2</mi></msqrt></math>is displayed as
With MathML being part of HTML5, all modern browsers support MathML. Which is a god-given for those of us who remember the HTML documents that were generated from LaTeX source files by converting formulae to bitmap graphics.