I would suggest looking at OxygenXML editor. It isnt open source, but its relatively cheap and has good HTML, css, etc support, and has built in support for webdav. I've found that editors which rely on OS webdav mounting arent as good as when its built in to the app.
Any text editor should do the job here - i'd recommend either Sublime Text 2 or Notepad++
As long as you have the WebDav path mapped as network drive you should be good to go.
I would suggest looking at OxygenXML editor. It isnt open source, but its relatively cheap and has good HTML, css, etc support, and has built in support for webdav. I've found that editors which rely on OS webdav mounting arent as good as when its built in to the app.