Winform webbrowser custom fonts not showing

I'm using the C# WebBrowser control (Winforms) and am passing markup into it via the .DocumentText property.

The document links to some css that uses the @font-face rule, which works when running locally (and from a server), but not when being consumed by the webbrowser control.

The css is pretty basic, it looks like this:

@font-face {
    font-family: FontName;
    src: url("/fonts/fontname.ttf") format("truetype");
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

I've tried the various Registry suggestions from over here (to no avail): https://weblog.west-wind.com/posts/2011/May/21/Web-Browser-Control-Specifying-the-IE-Version#FeatureDelegationviaRegistryHacks

Another common suggestion is to include a meta tag in the mark-up (which I have done) like this:

<meta http-equiv="X-UA-Compatible" content="IE=edge">

My font is already marked as installable (and works fine in IE), so there is obviously something specific about the webbrowser control that's the problem - but having spent hours trying to figure out, I'm at a loss as to what that could be.

Does anyone know what I'm missing here?

Thanks!