Hot answers tagged guidelines
Aside from the case that has been made for improved readability, I also argue for sentence case it on the grounds that it's an easier rule to remember for people actually implementing (graphic designers, engineering, writers, etc.). Title case lends itself to all kinds of arbitrary decisions when implementers don't want to be bothered to look up whether ...
Title Case for Headings and Buttons It's easier and faster for users if they can to identify the shapes of words. "We recognize words from their word shape." also called the Bouma Shape. Read more: http://en.wikipedia.org/wiki/Word_recognition Bouma Shape: http://en.wikipedia.org/wiki/Word_recognition#Bouma_shape A Few Examples ...
Why not create a PSD of your UI elements. For example this is the UI Guide created by Teehan+Lax for the site Medium.
Title case for buttons. As @juan-lanus suggests "US English is more capitalization-prone than Spanish.". As @bart-gijssens suggests there are no widely followed guidelines for webapps, but with Apple's attention to design and their significant share on mobile, I go with title case (title-style) for buttons as described in both iOS (pdf) and Mac HIG (pdf).
Only top voted, non community-wiki answers of a minimum length are eligible