“Markdeep Features: Multiple Columns”, Morgan McGuire (CSS, typography)⁠:

You can use the CSS columns style to make an HTML multicolumn block. Then, just use regular Markdeep within it and the browser will automatically apply your multicolumn layout…

multi-column only works well if you know that you have very short sections (as in this example), or if you were planning on printing to separate pages when done.