pb facs images

[eduarddrenth]

the TEI pb can have a @facs for a page facsimile, used by default in teipublisher for example.

This css can be added as an alternate stylesheet in author mode to show these facsimiles:

pb[facs] {
    content: attr(facs,url);
    width: 600px;
    display: block
}

Wanted to create a PR but couldn't find exactly where to place this css and how to configure it as an alternate stylesheet.