Revisit header font sizes, vertical spacing, content width

• Is the h3 font size too close to h2, dampening the visual hierarchy?
• Is vertical spacing around headers too tight, making the text harder on the eyes and more difficult to scan? Particularly h2 and h3
• May be a bug: there is no vertical space between h1 and the description when it's shown. I'd try 8-16 px
• Is the header description font size too small? Reference designs (Nature, New York Times, Distill, and Tailwind) consistently used a slightly larger size than the base font size. I'd try 19px
• With the base font size reduced from 20px to 17px, is the content width too wide? On a large display, reference designs are 600-750px, and Quarto is ~850px.

Suggestion

Here is a side-by-side comparison after making some of these changes. Left is original. On the right, I have:

• Decreased h3 font-size from 23px to 20px
• Increased h2 top and bottom margins from 34 17 to 49 24
• Increased h3 top and bottom margins from 25.5 8.5 to 40 20.

cc @allenmanning @dragonstyle

The issue names no files, tests, or entry points. Review the existing header typography and layout styles, compare h1–h3 spacing, description sizing, and content width against the references and screenshot, and confirm the intended values with maintainers. Done means an agreed, consistent visual update.