Otherwise known as Cascading Style Sheets.
What questions do you want to ask a Software Engineer about Frontend Web Development? I will try to answer your questions to help you get started.
Using -webkit-text-fill-color can create a knockout text effect, and render a gradient as text color. Consider this edge case with Flexbox and knockout text.
CSS Gradient Borders are possible with this one technique. You can even create CSS gradient border animations with a little CSS magic.