Flow boards set the h1 page heading to font-family: Georgia, color: #1E1E1E, with no bottom border. They also set h2 outside topics to color: #231f20 with no bottom border. But then the Typography refresh Beta feature sets both h1 and h2 to font-family: "DejaVu Serif", Georgia, serif. It's confusing: it suggests titles of Flow boards will appear different from regular pages when the font change will be minimal when Vector gets the Typography refresh; templates in a Flow board header appear differently than on regular pages; if you use a Flow board to discuss the Typography refresh you get slightly different fonts.
If we think changing header typography is a generally good idea, we should do it globally in Vector; if we think Flow topics and the Flow header need different typography than the default we should give them targeted selectors (as modules/discussion/styles/topic.less already does for the topic title) rather than redefining generic tags, and be sure to say why in the comments. Dropping the bottom border from h1 is an aesthetic choice that won't work when we use a <div id="contentSub"> under the heading line to improve navigation back to a board.
I think Matthias added global h2 styling for "Historical view of a topic"; if we really need this it could be a specific .flow-history-log h2 selector.
See also bug 59155.