ReSharper - ASPX tags invalid?

Hi

I am taking a look at Resharper for the first time. I am using it with an ASP.Net Web Forms project but notice it seems to highlight pretty much every single line on an ASPX page as having issues.  For instance take this one line:

<asp:label runat="server" text="Label"></asp:label>

It says "asp:label Unrecognised tag or prefix" and suggests that I want to remove the "runat" attribute.

Is there something that has to be configured for Resharper to understand ASPX files?  This is with VS 2017 and the latest download of Resharper 2017.3