You look up this new syntax you've discovered, and learn that it means a tag is "self-closing". At this point, why wouldn't you assume is self closing too? Or that is valid? Given this, I'm particularly sad that MDN uses self-closing tags in its beginner-facing documentation. ...
Self-closing mixerdoi:EP0439403 A1EP
Two ways of closing cracks on asphalt concrete pavements: Microcapsules and induction heating. In Key Engineering Materials; Trans Tech Publications Ltd.: Bäch, Switzerland, 2010; Volume 417, pp. 573–576. 4. Su, J.F.; Wang, Y.Y.; Han, N.X.; Yang, P.; Han, S. Experimental ...