In case you need some cool and beautiful background for the web but don’t need some complicated graphics on it, gradient background might be something you want for the web.
WebGradients is a free service that provide more than 180 linear gradient backgrounds, you can use for anything on the web.

Just hover any gradient, you can click on Copy CSS to get the code for styling, for example: when I copy CSS3 of 002 Night Fade, this is the styling.

background-image: linear-gradient(to top, #a18cd1 0%, #fbc2eb 100%);

Besides CSS3, you can download preset of linear gradient backgrounds in PNG, PSD and Sketch format, in case you want to apply into some graphic design.