How to do alt text on Ao3?

I'm in the middle of writing a fanfiction, and I'm using something called the thunderstorm-rainbow code. Essentially, it's a visual cue that indicates the beginning and end of a triggering/especially dark scene. I use a thunderstorm emoji to indicate the beginning of the triggering scene, and a rainbow emoji to indicate the end of the scene. Problem is, however, if somebody is visually impaired, or otherwise uses a text-to-speech device, the emojis might read weird. I've thought about using a word cue alongside the emojis, but that wouldn't help, so instead, I want to do alt text to my emojis, but I don't know how. Does anybody know how I can alt text things in my work, especially emojis? Thank you.