-webkit-tap-highlight-color

This webkit only CSS property is available on mobile Safari in both iOS and Android. It allows you to override the yellow highlight colour when a user taps a link or JavaScript clickable element. 

This is a useful accessibility feature that lets users know a link or clickable action has been touched rather than just tapping on the main content area (as you would to scroll the page). 

Occasionally this can interfeer with the design of a page, such as an image gallery, where the tap is more obvious and the change instant. To disable to tap highlight use the CSS:

-webkit-tap-highlight-color: rgba(0,0,0,0);

Where the important part is setting the opacity to zero (and therefore hiding the effect).