The "Create another account" link is a weird Frankenstein element that is a link inside an h3 inside a div that is assigned the mw-ui-button class to make it appear as a button. This causes all sorts of conflicting CSS to be assigned to it. For example, on mobile all h3s are displayed in serif font. (See screenshot.)
The h3 tag should be removed, as both the div and the link already have unique IDs available to increase the font-size (mw-createaccount-another and mw-createaccount-join).