I agreed with Chris that developers prefer to develop skins using ASCX approach.
Beside it's easy for them to make changes without having to reparse the HTML, it also allow them to take full control in skin objects customization without creating the skin.xml file. When installing skins, dnn essentially marges the skin.html and skin.xml to produce the skin.ascx with specific ASP.NET user controls and skin object attributes.
In brief:
Building HTML skin, common files that need to create:
- skin.html
- skin.doctype.xml (if using XHMTL/CSS)
- skin.css
- skin.xml
Building ASCX skin, common files that need to create:
- skin.ascx
- skin.xml (if using XHMTL/CSS)
- skin.css
Both approach is the right way to do, however, when develop commerical skins, it is better to take the HTML approach because it reaches the broader target audiences.
Hope this helps!