How to route EVERYTHING other than Web API to /index.html

Use a wildcard segment:

routes.MapRoute(
    name: "Default",
    url: "{*anything}",
    defaults: new { controller = "Home", action = "Index" }
);

in my case none of these approaches worked. i was stuck in 2 error message hell. either this type of page is not served or some sort of 404.

url rewrite worked:

<system.webServer>
    <rewrite>
      <rules>
        <rule name="AngularJS" stopProcessing="true">
          <match url="[a-zA-Z]*" />

          <conditions logicalGrouping="MatchAll">
            <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
            <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
            <add input="{REQUEST_URI}" pattern="^/(api)" negate="true" />
          </conditions>
          <action type="Rewrite" url="/" />
        </rule>
      </rules>
    </rewrite>
    ...

notice i matched on [a-zA-Z] because i don't want to rewrite any of the .js .map etc urls.

this worked in VS out of hte box, but in IIS you may need to install a url-rewrite module https://www.iis.net/downloads/microsoft/url-rewrite

Suggest more native approach

<system.webServer>
    <httpErrors errorMode="Custom">
        <remove statusCode="404" subStatusCode="-1"/>
        <error statusCode="404" prefixLanguageFilePath="" path="/index.cshtml" responseMode="ExecuteURL"/>
    </httpErrors>
</system.webServer>