16 × 16 · canvas · png
Draw the little picture that sits in a browser tab, one pixel at a time. A smiley is already loaded — paint over it, and watch the 16-pixel copy land in a tab mock-up on the right.
Drag to draw. B paint · E erase · G fill · I pick · S mirror · Ctrl+Z undo. Click the artboard first, then arrow keys move and Enter paints.
A tab favicon is 16 pixels across — the same size as the leftmost square. That is the whole reason pixel icons look the way they do.
A pretend browser window, drawn on this page — the icon in the tab is your artboard, live.
A tab strip is either near-white or near-black, and you get no say in which. This counts the share of your icon that stands clear of each one — 3:1 contrast or better.
Nothing saved yet. Every size is drawn from the same 16 × 16 grid with smoothing switched off, so the edges stay hard.
Paste this straight into an <img src>
or a <link rel="icon"> — no file needed.