Perhaps time to add it to the FAQ.

It certainly is a frequently asked question. But spelling everything out is against the spirit of the site.

On HN it's good for readers to have to work a little, and the audience is smart enough to figure things out.

It's equivalent to the flag at half mast:

> When someone important to the community dies, a thin black bar is added to the top of HN as a mark of respect

Welcome to HN! I love this question every time I see it.

Paul Allen, co-founder of Microsoft, has died.

It's for the late Paul Allen RIP. Should've known, sorry all.

I'm relatively new to HN, and as such I hadn't encountered the black line before. Thanks for asking the question so that noobs like me can find out the answer.