HTML: use CSS flex boxes for columns.

This allows to render an arbitrary number of columns, while the previous
approach assumed exactly two columns.

test/s5-basic.html

@@ -13,7 +13,8 @@
   <style type="text/css">
     code{white-space: pre-wrap;}
     span.smallcaps{font-variant: small-caps;}
-    div.column{display: inline-block; vertical-align: top; width: 50%;}
+    div.columns{display: flex; gap: min(4vw, 1.5em);}
+    div.column{flex: 1;}
     div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
     ul.task-list{list-style: none;}
     .display.math{display: block; text-align: center; margin: 0.5rem auto;}