When styling headlines (H1-H6 tags) in your CSS, you should know that Presenter sometimes sets the margins for headlines as a part of its layout process. This happens individually for every headline and overwrites your CSS margins.
Presenter assigns overwriting margins to headlines in the following two scenarios:
If the “Center slides vertically” option is active
If this option is active AND the headline is the only element on the slide, the top and bottom margins are set to 0.
But why? This is done because the headline would not be perfectly centered on the slide if the margins stay - the margin would push the headline a little above the vertical center, making the slide look uneven.
If the “On slides with title, fix title on top and center content” option is active
If this option is active and there are more elements on the slide, Presenter uses the bottom margin of the headline to create the space between the headline and the content.