Well, it took about a decade for web standards to become a real thing and a lot longer for Web Platform Tests to come to be. Still, while there are lots of tests for DOM construction and visual rendering, testing construction of the accessibility tree is lacking (also keyboard interaction testing).

And that's just for browsers, there's no shared spec for the operating system accessibility APIs the browsers' accessibility tree has to be translated into or how screen readers (and other assistive technologies) will use the OS's APIs.

Granted, I do not know what I am doing with CSS, but the Character Sheet example seems standard flexible elements?

Some of the extracted CSS chunks

  #statblock{
    box-shadow:0 20px 25px -5px rgba(0,0,0,.1),0 10px 10px -5px rgba(0,0,0,.04);
    font-family:Lato,'Trebuchet MS',sans-serif;
    font-size:85%;
    min-width:50ch;
    max-width:70ch;
    margin-inline:auto;
    background-color:#fffaf0;
    padding-inline:2rem;
    padding-block:1rem
  }
  dl.statblock-bio{
    color:maroon;
    line-height:1.5;
    border-top:5px solid maroon;
    border-bottom:5px solid maroon;
    margin-block:0.75em;
    padding-block:0.75em
  }
  dl.ability-scores{
    min-width:40ch;
    display:flex;
    justify-content:space-around;
    color:maroon
  }
  dl.ability-scores>div{
    text-align:center;
    line-height:1.5
  }
  dl.ability-scores dt{
    font-weight:700
  }

I've found CSS Grid is extremely useful for styling DLs.