[Skins] Error in code-view: No build provider registered for the extensions '.skin'

In the code-view of a skin file the first line has a blue line under it, with the following tooltip:

ASP.NET runtime error: There is no build provider registered for the extensions '.skin'. You can register one in the <compilation><buildProviders> section in machine.config or web.config. Make sure is has a BuildProviderAppliesToAttribute attribute which includes the value 'Web' or 'All'.

What does this mean?

PS. Typo ("...Make sure is has...") is not mine.