Both. REM for general typographic rules. EMS for relative specific sizing.

.lego { display: block; }

The best thing is export to PDF, at least it's scalable in AE :)

Probably these:

<title></title> <description>Created with Sketch Beta.</description> <defs></defs>

And this:

xmlns:sketch="http://www.bohemiancoding.com/sketch/ns"

I think @pumpkin is the man who did it ;)