All the executable content elements we’ve discussed so far have had one common trait: they are separate from the browser and the HTML/XHTML document — separate data, separate execution engine.
document is via the HTML and XHTML standard
</script> is processed by the browser as
XHTML within this tag; it is flagged as an error by the browser.
However, browsers that do not support
<script> process its contents as regular HTML, to the confusion of the user. For this reason, we recommend that you include the contents ...