CSS

Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language such as HTML or XML.

Background rainbow gradient.

background: linear-gradient(90deg, rgba(255,0,0,1) 0%, rgba(255,154,0,1) 10%, rgba(208,222,33,1) 20%, rgba(79,220,74,1) 30%, rgba(63,218,216,1) 40%, rgba(47,201,226,1) 50%, rgba(28,127,238,1) 60%, rgba(95,21,242,1) 70%, rgba(186,12,248,1) 80%, rgba(251,7,217,1) 90%, rgba(255,0,0,1) 100%);