Text fading out is a CSS gradient effect that leverages certain webkit properties, thus it only works in browsers that support webkit properties. This includes blink based browsers since they are a fork of webkit. Between webkit and blink, that covers the most popular browsers people are using these days but there remain a number that do not support webkit properties, especially among mobile browsers.
The above linked gradient example is for a single line of text, but the concept can be applied vertically to a block of text as well. The paragraph you want to fade out should have a unique class attribute. You can the style that class with custom CSS added to the Additional CSS section of the customizer or style book.
