As far as the display problem on Firefox and Safari on Mac goes, I've done some experimenting. It looks like it is a font thing. HN has "font-family:Verdana, Geneva, sans-serif;" as part of the style sheet.

I made a minimal test page with just the different space examples, and an internal style sheet that just set the font-family for the body, and it shows the problem. Changing it to "Arial, sans-serif" makes it work.

I've never looked seriously into fonts for browsers, so have no idea what is going on at this point. If there is a problem with the Mac versions of Verdana and/or Geneva, then why is it only affecting Firefox and Safari, and not Chrome, on my Mac?

If anyone else wants to try to figure this out, here is the test page I've been using:

  <!DOCTYPE html>
  <head>
      <meta charset="utf-8"/>
      <title>space test</title>
      <style>
          body { font-family:Verdana, Geneva, sans-serif; }
          xbody { font-family:sans-serif;}
          xbody { font-family:Arial, sans-serif;}
      </style>
      
  </head>
  <body>
  <p>
  1&#x200B;234&#x200B;567 (U+200B, ZERO WIDTH SPACE)<br/>
  1&#x200A;234&#x200A;567 (U+200A, HAIR SPACE)<br/>
  1&#x202F;234&#x202F;567 (U+202F, NARROW NO-BREAK SPACE<br/>
  1&#x2009;234&#x2009;567 (U+2009, THIN SPACE)<br/>
  1&#x2006;234&#x2006;567 (U+2006, SIX-PER-EM SPACE)<br/>
  1&#x2008;234&#x2008;567 (U+2008, PUNCTUATION SPACE)<br/>
  1&#x2005;234&#x2005;567 (U+2005, FOUR-PER-EM SPACE)<br/>
  1&#x2004;234&#x2004;567 (U+2004, THREE-PER-EM SPACE)<br/>
  1&#x0020;234&#x0020;567 (U+0020, SPACE)<br/>
  1&#x2000;234&#x2000;567 (U+2000, EN QUAD)<br/>
  1&#x2002;234&#x2002;567 (U+2002, EN SPACE)<br/>
  1&#x2007;234&#x2007;567 (U+2007, FIGURE SPACE)<br/>
  1&#x2003;234&#x2003;567 (U+2003, EM SPACE)<br/>
  1&#x2001;234&#x2001;567 (U+2001, EM QUAD)  </p>
  </p>
  </body>