CSS Margin only works on chrome

I have a little website, and I wanted to have no margin by doing:

<p id="demo" style="margin-block-start:0em;margin-block-end:0em"></p>

But the object still has a margin on every browser except chrome. (Chrome on IOS also does not work) What have I done wrong? I have seen some other problems that were related to some small typos, but I can't find any of that here.