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This is an excerpt taken from DotNetNuke Skinning 101:
The problem in Internet Explorer is that my skin is being rendered using HTML 4.0 transitional. IE8 is rendering this page in quirks mode which doesn’t properly center the content. What I really want is for the skin to be rendered in XHTML 1.0 Transitional. I could have the administrator set the default rendering in DotNetNuke to XHTML 1.0 Transitional, which is not really a viable solution. A well behaved skin will just work and not require effort on the part of the administrator to get it to look proper. The other solution, and the one that is recommended is to include an XML file with our skin which tells the rendering engine the appropriate DOCTYPE for the skin. In my current example, I have named my layout “Index – 1.html” so I will create a new file called “Index – 1.doctype.xml” and place it in the same folder as my layout file. The doctype file includes the following XML content.
This file allows me to independently set the doctype for each layout in my skin, regardless of the setting in DotNetNuke.
The author of this tutorial creates an original index.html file. He then says an additional index-1.html file is created, along with an index-1.doctype.xml file. The files are placed in the Skins folder.
My question is, how does DNN know when to use index-1.doctype.xml when the user is using IE8, and not using it when the user is viewing the page in IE9 or later? Right now I have one ascx file with the html. I'm not sure if I am supposed to have two different files for different doctypes.