Steps:

1. Enable JavaScript preview.
2. To an empty page add something in a source tag e.g.

<source lang="css">
li[id^="pt-prefswitch-"],
li[id^="pt-optin-"]
{display:none !important}
</source>

3. Run preview.

Expected:
Same preview as without JS.

Result:
The text seem to be unparsed.

This is because geshi extension produces CSS for the language inside the generated page and JS preview. You might say this is a bug in geshi, but geshi works fine without JS and there might be more cases like that with other extensions.

Version: 1.16.x
Severity: normal