R presentation section headers look way fatter and more bold when viewed in browser vs. preview

Hi @sjp! Would you be able to make a simple reprex, or reproducible example, for this? Even if it's just a default template and a few screenshots of the rendering in different browsers. Text rendering can be surprisingly different across browsers and devices, and looking at the results might help us work out if this is an R Markdown-specific issue or just down the way CSS is variously rendered :slight_smile: