Create perfect squares or circles and preserve aspect ratio when resizing and holding shift. Now in marker.js v3.5.
markerjs.com

marker.js always had a rectangular highlight marker but the one resembling a physical highlighter was not on the menu. That changes in v3.2.

Curve marker somehow slipped through the cracks and didn't make it into the 3.0 release. Oops 🤭 Fixed in v3.1 out now.

markerjs.com