Ask HN: Can one add an HTML comment before the DOCTYPE?

In HTML, the first line should be the DOCTYPE: <!DOCTYPE html>, so that the client knows it's HTML. The fact that the line starts with `<!` tells the client that what follows is not actual code to be interpreted.

So, I'm wondering: can one add comments before the DOCTYPE? Will they be ignored, will they trigger compatibility mode on some browsers, or we just don't know?

I'd like to do:

<!-- this is a comment --> <!DOCTYPE html> <head> etc.