Using container as custom media queries

Do you see any problem handling custom media queries without the need of postcss preprocessor?

I thought something like this could do:

```
:root {
--is-below-xxs: false;
--is-below-xs: false;
--is-below-sm: false;
--is-below-md: false;
--is-below-lg: false;
--is-below-xl: false;
--is-below-xxl: false;

@media screen and (max-width: 240px) {
--is-below-xxs: true;
}
@media screen and (max-width: 360px) {
--is-below-xs: true;
}
@media screen and (max-width: 480px) {
--is-below-sm: true;
}
@media screen and (max-width: 768px) {
--is-below-md: true;
}
@media screen and (max-width: 1024px) {
--is-below-lg: true;
}
@media screen and (max-width: 1440px) {
--is-below-xl: true;
}
@media screen and (max-width: 1920px) {
--is-below-xxl: true;
}
}
```

Usage:
```
@container style(--is-below-xxs: true) {
/* ... */
}
```

I've checked the browser compatibility, and it seems fine: https://caniuse.com/?search=%40container

What do you think? I can try to make a pull request if you think it would be a good fit for the project.

No project file, test, or entry point is named. Start by reviewing the proposed custom media-query approach and its browser-compatibility reference, then determine the project scope and acceptance criteria. Done would require an agreed implementation and validation path.

Written by the indexing model from the issue text.