CSS Level | Value |
---|---|
CSS 2.1 | "[ <cue-before> || <cue-after> ]", where <cue-before> and <cue-after> is a URL or "none" |
CSS3 | "[ <cue-before> || <cue-after> ]", where <cue-before> and <cue-after> is "<uri> [<number> | <percentage> | silent | x-soft | soft | medium | loud | x-loud] | none" |