This works for both mobile and non-mobile. The crux is that text-indent: -20 will outdent the first line by 20 px. The margin setting ensures that the text doesn't accidentally overlap something else on the page.
If the screen is small enough and any lines wrap, text-indent will not impact any subsequent lines causing them to appear indented, which is the poetry convention for a wrapped line.
This is web-only, and not designed for e-readers. I started with an article on that here, but ultimately the first-line stuff led me astray for a while. For my purposes, text-indent alone was enough.