Add an explanation for the following css declaration syntax to the docs.
```

"Hello World!"
css bg:{item.color}
```
is the same as
```
"Hello World!"
```

No documentation file or test is named in the issue. Locate the relevant CSS syntax documentation, add an explanation showing that the nested declaration and bracketed declaration are equivalent, and verify that both examples render clearly in the docs.

Written by the indexing model from the issue text.