It appears to highlight the letters that your guesses have narrowed it down to. For example if you had narrowed it down to after 'sudden' and before 'super', it would highlight 'su'.
> The other party can use whatever client they have to connect to port 8000 of my system, e.g., a web browser, nc HOST 8000, curl HOST:8000, or even, ssh HOST -p 8000, irssi -c HOST -p 8000, etc.
Now I feel a bit like writing some tool to automatically follow your posts (easy with existing HN replies) do a semantic analysis on them to determine when you will make that misstake again and give some alert (the hard/expensive part).
But then somewhat spoiling the page's nice light weight is a 576kB favicon.