Symptoms
The website is running in IIS 7+ and .HTML pages won’t render but .HTM will.
Cause
A HTML page handler in the web.config line is causing the issue.
Solution
Comment out the following line in the web.config file in the <handlers> section:
<add name="HtmlPages" path="*.html" verb="*" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v2.0.50727\aspnet_isapi.dll" resourceType="Unspecified" preCondition="classicMode,runtimeVersionv2.0,bitness32" />