The explainer lists the following goal:

Identical appearance on all platforms and all supported browsers by default

but provides no justification for it. Existing form controls have either a platform-specific or a browser-specific look by default. Some also de facto have the ability to remove that look via custom styling (e.g. <input type=text> or <input type=button>), while others can't be freely custom styled in a cross-browser way (e.g. <input type=checkbox>). The ones that do allow free styling tend to still be used in contexts that demand full custom looks, while the latter are replaced with div soup.

I think a default platform/browser/etc specific look that is unspecified, plus full styling with CSS that can entirely replace the look, is the best target. That's the most successful kind of form control so far, and it looks good both in mildly styled websites that don't care to override all form controls, and in heavily styled sites that want a total custom look.

It's not clear why a default cross-browser cross-platform look (but which might still not be fully specified per issue #20 ) would be better. Even if it looks good on lots of platforms and websites today, it will eventually look dated if frozen. And if not frozen, it can't achieve the goal of looking exactly the same on all browsers and platforms. On the other hand, a platform/browser specific default look can be updated over time.