Use pseudo-class :contains-processing-instruction(target, ..., target) where target, a CSS identifier or string, is the target of the processing instructions.

Several processing instruction targets may be specified, in which case, the :contains-processing-instruction() test will pass if the subject of the test directly contains a processing instruction having any of the specified targets.

Example: display all XHTML spans containing one or more marker processing instruction with a yellow background.

span:contains-processing-instruction(marker) {
    background-color: yellow;
}