Anyone who regularly works on HTML emails will know that they can be pesky beasts. Things like a strong call to action button are extremely important for fundraising appeals and event registrations, but it can be hard to get them looking nice. Relying on images is not a good idea, as many subscribers will have image blocking turned on.
This useful page from SendWithUs has some ‘bulletproof’ email components, that have been tested in “all email clients that matter”. It includes attractive rounded buttons and image captions.