Macro: arc symbol under, over

From Arc Circle Syntax.

A macro to create an arc over or under content (using parenthesis for lack of alternatives).

As part of the MathJax configuration:

<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
    TeX: { Augment: {
      Definitions: {
        macros: {
          overparen: ['UnderOver','23DC'], // UnderOver represents the MathML element, 23DC the unicode character
          underparen: ['UnderOver','23DD'] // UnderOver represents the MathML element, 23DC the unicode character
        }
      }
    }}
  });
</script>

Then $\overparen{XYZ}, \underparen{XYZ}$ will provide the desired effect.

Outside of a configuration file

If you don't have access to a configuration file (on sites like stackexchange), you can use the following (possibly as part of a \newcommand).

\overset{\mmlToken{mo}{⏜}}{XYZ}
\underset{\mmlToken{mo}{⏝}}{XYZ}

You can replace 23DC and 23DD with your favorite unicode characters.