url() - CSS: Cascading Style Sheets
occur inside single quotes unless escaped. the following are all valid and equivalent: <css_property>: url("https://example.com/image.png") <css_property>: url('https://example.com/image.png') <css_property>: url(https://example.com/image.png) path references the id of an svg shape -- circle, ellipse, line, path, polygon, polyline, or rect -- using the shape's geometry as the path. ...
...s the id of an svg shape -- circle, ellipse, line, path, polygon, polyline, or rect -- using the shape's geometry as the path. <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. ...

CSS data types - CSS: Cascading Style Sheets
see also css units and values introduction to css: values and units ...