It's different for the different header levels.

https://www.w3.org/Provider/Style/GuideStyle.css

    H1 { margin-left: -20%; }
    H2 { margin-left: -15%; }
    H3 { margin-left: -10%; }
    H4 { margin-left: -05%; }

I find it works well for helping maintain context. I often can't easily tell the difference between different headers unless they're right next to each other, and even then some of them are difficult to discern. Adding the indentation is a way to make the hierarchy more clear.