This is
an HTML5 page
for me to have fun with. It is intended to be syntactically valid
both as HTML (i.e., `text/html`

) and
as XHTML (i.e., `application/xhtml+xml`

): see
the recommendation
on polyglot markup
for details (although this is not exactly polyglot markup because of
the way the inline stylesheet is escaped).

Everyone should know that $2+2=4$ and that ${\int}_{-\infty}^{+\infty}{e}^{-{x}^{2}}dx=\sqrt{\pi}$.

This paragraph is *editable*. By
clicking on it (or otherwise selecting it) you should be able to
modify it through your web browser.