The goal is: write some CSS so that if an article contains nothing except <img class="emoji">, and it contains four or fewer of them, then make those emoji triple size.

The question is: is this doable with pure CSS?

Answer: yes. Yes it is. It needs a heroic selector, though:

.emoji:nth-child(1):nth-of-type(1):nth-last-child(1):nth-last-of-type(1), /* 1 of 1 */
.emoji:nth-child(1):nth-of-type(1):nth-last-child(2):nth-last-of-type(2), /* 1 of 2 */
.emoji:nth-child(2):nth-of-type(2):nth-last-child(1):nth-last-of-type(1), /* 2 of 2 */
.emoji:nth-child(1):nth-of-type(1):nth-last-child(3):nth-last-of-type(3), /* 1 of 3 */
.emoji:nth-child(2):nth-of-type(2):nth-last-child(2):nth-last-of-type(2), /* 2 of 3 */
.emoji:nth-child(3):nth-of-type(3):nth-last-child(1):nth-last-of-type(1), /* 3 of 3 */
.emoji:nth-child(1):nth-of-type(1):nth-last-child(4):nth-last-of-type(4), /* 1 of 4 */
.emoji:nth-child(2):nth-of-type(2):nth-last-child(3):nth-last-of-type(3), /* 2 of 4 */
.emoji:nth-child(3):nth-of-type(3):nth-last-child(2):nth-last-of-type(2), /* 3 of 4 */
.emoji:nth-child(4):nth-of-type(4):nth-last-child(1):nth-last-of-type(1) {/* 4 of 4 */
  width: 3em; height: 3em;
}

I am some leading text

I am some trailing text

I am some middle text