As c69 mentioned there is no way to do it with pure CSS.
but you can use HTML instead:
use
<head>
<base target="_blank">
</head>
in your HTML <head>
tag for making all of page links which not include target
attribute to be opened in a new blank window by default.
otherwise you can set target attribute for each link like this:
<a href="https://stackoverflow.com/yourlink.html" target="_blank">test-link</a>
and it will override
<head>
<base target="_blank">
</head>
tag if it was defined previously.