There are lots of IDEs & editors out there. Free and commercial.
For JavaScript, Aptana ROCKS!
http://www.aptana.com/However, for XML, you won't find a WYSIWYG editor. It's simply not possible unless you start bending the rules & definitions there. XML is for storing data in self-describing tags. Now, what are those definitions? Well, anyone can make up any kind of definition that they want. For example, here's a short snippet from a GDT file (it's an XML file I use for my own software):
<?xml version="1.0"?>
<RenegadeGDT>
<Version Major="4" Minor="0" Build="0" />
<Volume>12.1907994151115</Volume>
<Pan>0</Pan>
<Speed>0</Speed>
<PitchShift>0</PitchShift>
<CountIn OnOff="False">10</CountIn>
<Gain>0</Gain>
<ZoomDistance>100</ZoomDistance>
<PadLoopZoom>0.6</PadLoopZoom>
<SkipBack>-5</SkipBack>
<SkipForward>5</SkipForward>
<SpeedTrainerOn>False</SpeedTrainerOn>
<SpeedTrainerIncrement>5</SpeedTrainerIncrement>
<SpeedTrainerTimesToLoop>1</SpeedTrainerTimesToLoop>
<SpeedTrainerStartSpeed>-50</SpeedTrainerStartSpeed>
<SpeedTrainerEndSpeed>50</SpeedTrainerEndSpeed>
<LoopItBaby>True</LoopItBaby>
...
<EQSettings Count="2">
<EQSetting Name="Zero" Selected="True">
<Band BandIndex="0" Bandwidth="0.3333333" Q="0" Center="18.35405" Gain="0.0" Channel="Left" />
<Band BandIndex="0" Bandwidth="0.3333333" Q="0" Center="18.35405" Gain="0.0" Channel="Right" />
<Band BandIndex="1" Bandwidth="0.3333333" Q="0" Center="23.12465" Gain="0.0" Channel="Left" />
<Band BandIndex="1" Bandwidth="0.3333333" Q="0" Center="23.12465" Gain="0.0" Channel="Right" />
...
I'm the one that wrote the spec there, but how should it be "displayed"? Well, there is no display for it unless I write up an XSLT, which is a processing directive for XML that can be used to output the XML in a more readable format, like HTML. (Which is what I meant by bending the rules above.)
Now, there are XML editors, but they're really just text-based, and not "visual" designers.
Not sure if any of that is useful for you or not.
I'll leave recommendations for specific HTML editors out there to someone else as the tools that I'd recommend probably are right for what you need.