I'd rather go with a single attribute that contains all once identifiers:

```html
<div data-jquery-once="foo bar baz">
```

...which appears to be what @theodoreb's proposed code is doing already?

If people really want to style elements dependent on .once processing, then they can still do so:

```css
[data-jquery-once*="bar"] {
  font-weight: bold;
}
```
Has a small risk of false-positives, but usage of unique and/or properly namespaced .once identifiers is not the responsibility of the library, but rather a user-space policy decision.