OnDoc/packages/Microsoft.AspNet.Web.Optimization.WebForms.1.1.3/content/web.config.transform

<configuration>
  <system.web>
	<pages>
    <controls>
      <add assembly="Microsoft.AspNet.Web.Optimization.WebForms"
           namespace="Microsoft.AspNet.Web.Optimization.WebForms"
           tagPrefix="webopt" />
    </controls>
	</pages>
  </system.web>
</configuration>