The favicon is the only version of your mark most people will ever see. It loads before the page, it sits in the tab for hours, and it scales down to a 16 × 16 grid that ignores almost every technique that works at display size. If it doesn't read there, it doesn't read.
I designed this site's mark the other way around: from the favicon outward. Five tiles on a 3 × 3 grid — a blue square, a green square, a yellow sun, a purple core, an orange brick. Each tile can hold its ground at 16 pixels. Scaled up, they still read as the same thing.
What the constraint taught me
Shapes beat letterforms at this size. A letterform that survives 16 × 16 is either hollowed out or so bold it loses all its character. Geometric shapes — circles, squares, triangles — don't have that problem. They're the same shape no matter the resolution.
Count before you color. Five shapes is already the upper limit. Six or seven, and you can't tell one from another at small sizes. The palette followed from the number of tiles, not the other way around.
A logo isn't only a logo. Those five tiles now appear in the footer as a row of small punctuation marks, as the period after each page title (triangle for Pages, square for Design, circle for Life), and as the category color on the sidebar links. The mark disassembles into a system. That only works because each piece was strong enough on its own.
If I had to do it again
I'd still start at 16 × 16. Every time I've tried to design big-first and scale down, the mark collapses at small sizes and I end up making a second one that doesn't look related. Easier to do the hard version first.