@cchaos onClick is covered by combining with ButtonHTMLAttributes further down in the file

@chandlerprall thanks, updated the changelog, removed the requirement for onClick prop since an href without an onClick is totally valid, and added isLoading prop to get them all in sync.

So, the latest types I supplied are somewhat undesirable because it changes the type of the onClick() prop to require a handler for React.MouseEvent<HTMLButtonElement | HTMLAnchorElement>, which really isn't necessary if people are relying solely on onClick() and using this as a standard button. I've tried a couple different ways of having TypeScript detect if you're using the href attribute and base onClick() on that, but I haven't figured out a way to do that which doesn't have other negative side effects... Going to try a couple other things but this will probably impact the default <EuiButton /> use case more than it's worth...

Okay, these types work for the following situations:

<EuiButton
  href="string"
  onClick={(e: React.MouseEvent<HTMLAnchorElement>) => undefined}
/>

<EuiButton
  onClick={(e: React.MouseEvent<HTMLButtonElement>) => undefined}
/>

<EuiButton
  href={optionalHref}
  onClick={(e: React.MouseEvent<HTMLButtonElement | HTMLAnchorElement>) => undefined}
/>