This is not a selector, but is used with the pseudo-elements ::before and ::after. There are only certain types of content that you can generate. Here is a rundown.
This is the content that will be placed either before or after elements:
content(none, <string>, <uri>, <counter>, open-quote, close-quote, no-open-quote, no-close-quote, attr(x))
Following are the parameters and their description:
none: This parameter does not generate any contentnormal: This is the default parameter and is the same as none<string>: This is any string text content<uri>: This will map to a resource, for example, an image<counter>: This can be used as either thecounter()orcounters()function to put a counter before or after each elementopen-quoteandclose-quote: This is used with the quotes generated content propertyno-open-quoteandno-close-quote: This does not add content, but increments or decrements the nesting level of quotesattr...