X-Develop does not have it yet. I know that there was some work on using MozEmbed for this function for MonoDev (IIRC on the SOC a few years ago), but was not integrated.
If you really need this functionality, then probably the best option is a copy of Windows (in the virtual machine) and MS Expression Web or Web Developer 2008 Express. This is either this, or learning how to write markup, which is usually the case with many developers.
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