lacks mark

[diomed]

I did smth like this:

mark 
{color: ghostwhite;
background-color: #ffcf30;
border-radius: 4px;
padding: 0.5%;}

would be good to put it in, or to make your own version, as you wish

[ruanmartinelli]

Hi @diomed, I don't use the mark element very often, but feel free to open a PR!