url() - CSS: Cascading Style Sheets
<url-modifier> in the future, the url() function may support specifying a modifier, an identifier or a functional notation, which alters the meaning of the url string.this is not supported and not fully defined in the specification.
... formal syntax url( <string> <url-modifier>* ) examples content property html <ul> <li>item 1</li> <li>item 2</li> <li>item 3</li> </ul> css li::after { content: ' - ' url(https://mdn.mozillademos.org/files/16761/star.gif); } result data-uri html <div class="background"></div> css .background { height: 100vh; } .background { background: yellow; background: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' width='90' height='45'%3e%3cpath d='m10 10h60' stroke='%2300f' stroke-width='5'/%3e%3cpath d='m10 20h60' stroke='%230f0' stroke-width='5'/%3e%3cpath d='m10 30h60' stroke='red' stroke-width='5'/%3e%3c/svg%3e"); } specifications specification status comment css values and units module level 4the definition of 'url()' in...

CSS data types - CSS: Cascading Style Sheets
custom-ident> <dimension> <filter-function> <flex> <frequency> <frequency-percentage> <gradient> <ident> <image> <integer> <length> <length-percentage> <number> <number-percentage> <percentage> <position> <quote> <ratio> <resolution> <shape-box> <shape-radius> <string> <time> <time-percentage> <timing-function> <toggle-value> <transform-function> <type-or-unit> <url> <url-modifier> <zero> specifications specification status comment css values and units module level 4 editor's draft css values and units module level 3 candidate recommendation initial definition.