- Links for the up/down arrows with target=<hidden iframe>, that set "visibility: hidden" on :visited. But oh, only color-changing CSS properties are allowed for :visited selectors.

- Links for the up/down arrows with unicode ▲ and ▼ that set their color to the page background color on :visited, using a sibling selector to hide the other arrow as well once either has been clicked. But oh, sibling selectors are forbidden in conjunction with :visited selectors. Same for selecting nested elements inside a :visited link - only the simplest uses of :visited are allowed. One could wrap both arrows in the same <a> tag, but then there'd be no differentiating between an up/down vote.

- An invisible radio button next to each up/down arrow, with both wrapped inside a <label> so that clicking the arrow causes the radio button to become :checked. Use a sibling selector to hide the arrows when either radio button becomes checked. But oh, if you click a link inside a <label> it's not counted as marking the radio button as selected.

- An invisible radio button hidden under each vote up/down image. Each radio button is wrapped in an <a> tag that causes a hidden iframe to navigate to the voting link. The vote up/down buttons are "pointer-events: none", so trying to click on one really marks the radio button underneath as selected and triggers a link click. This works fine in Firefox, but in Chrome, marking a radio button as selected doesn't trigger a link click.