Add support of element() and running()

[Bahar1978]

Dear,
I tried to load the content of an html tag with specific class via css in a @top-right of the header of the pdf file using weasyprint. Something like https://www.w3.org/TR/css-gcpm-3/#funcdef-running .
But it gives me a warning: Ignored 'content: element(heading) , invalid value . Also the same warning for poison: running(heading).
Could you please let me know if it is a supported feature or not?

Thanks in advance,
Kind regards,
Hajar

[Tontyna]

As the warning says: element() and running() not (yet?) supported by WeasyPrint.

[liZe]

Fixed by #882.