My problem doesn’t have to do with the <h1> tag, because from the images, you can see that the margin is outside the top .header element.
The problem I face is a default margin in the card template itself that forces this margin on all elements and I want to remove it. I was already able to remove this margin in Anki for Windows, but I was unable to remove it in Anki Droid.
I created a script to view the source code of the page on Anki Droid and I already found out which id has this margin applied. It’s the #content id.
#content {
margin: 0;
}