It turned out to be a conflict with the usage of the button tag:

<style>
.Main
{
   font-family: Raleway;
}
</style> 

<div class="Main">

<button>Sign in</button>

</div>

When the button tag is included in a page, which transfers contain into a parent window, a font which is not builtin will no longer be recognized.

This happens even is the font is properly declared in the parent window.

I will adjust to remove all button tag declarations and replace that with a different CSS approach.