add the custom font to your index.html aswel.
It is because the layout is loaded but the body uses another font. This is a small CSS thing of inheritance. So it will use the standard font untill you change your canvas size by expanding or minimizing your browser window. Or you put it on the body so it get's auto loaded.
I just did it for my game. undesign.be/X3
open your html file when exported.
add this line to your <head>
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<link rel="stylesheet" href="stylesheet.css" type="text/css" charset="utf-8" />
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Then add this to your Body{ } style
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font-family: <name of your font>;
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boom your font is there from the start of your game