Cannot parse the selector `.bg-\\[\\#86efac\\]`

The following css causes syntax error:

```css
.bg-\\[\\#86efac\\] {
  background-color: #86efac;
}
```

I think this is a valid selector. What do you think?

(I found this in [Fresh source code](https://github.com/denoland/fresh/blob/3900aa70d4b305080f42e5814bc832b22aee2d39/init/src/init.ts#L317-L319))