For the benefit of others. This should be included in the docs somehow.
I used NuGet to install Html Agility Pack into my Windows Phone project and it added HtmlAgilityPack.dll as a reference. However compiling gave the following error:
error CS0012: The type 'System.Xml.XPath.IXPathNavigable' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml.XPath, Version=126.96.36.199, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.
There is no such assembly provided for Windows Phone. However one can add a Silverlight version of that assembly, e.g. from
c:\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\Client\System.Xml.XPath.dll
Adding the reference gives a warning, see below. There is no error at runtime however and the code works are expected. (Presumably an app using it passes certification but I haven't published using this.)
We can see that it is referenced this way in the HapPhone.7.1.csproj project:
<HintPath>..\..\..\..\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\Client\System.Xml.XPath.dll</HintPath>