I do not know why, but by default the body element has an overflow of hidden set via css. There is probably a good reason to do this that I do not know about. Would love to know the reason behind that from a WebDev as it strikes me as unnecessary having seen the page with JS and without JS and not being able to see much of a difference.
Might be to avoid a visual reflow - otherwise the content will render, then the webfont will load and the whole page will reflow and jump around.
Google amp used to have a ridiculous 4 second timeout waiting for the font