# Beginner MDN curriculum CSS example not working in Firefox 158.0 (64-bit)

**URL:** <https://discourse.mozilla.org/t/beginner-mdn-curriculum-css-example-not-working-in-firefox-158-0-64-bit/149837>\
**Category:** MDN\
**Tags:** learning, issue\
**Created:** [October 16, 2026, 6:42pm UTC](https://discourse.mozilla.org/t/beginner-mdn-curriculum-css-example-not-working-in-firefox-158-0-64-bit/149837 "2026-10-16T18:42:44Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![schooldude777](https://avatars.discourse-cdn.com/v4/letter/s/4af34b/32.png) [@schooldude777](https://discourse.mozilla.org/u/schooldude777)\
**Post date:** [October 16, 2026, 6:42pm UTC](https://discourse.mozilla.org/t/beginner-mdn-curriculum-css-example-not-working-in-firefox-158-0-64-bit/149837/1 "2026-10-16T18:42:44Z")

</div>

Hello,

I’m currently at the section “Getting started with CSS” where they teach about [at-rules](https://developer.mozilla.org/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started#rules).

I’m trying the following code example:

```css
@media (width < 550px) {
  body {
    background-color: rgb(179, 255, 0);
  }
}

@media (width >= 550px) {
  body {
    background-color: rgb(255, 153, 0);
  }
}

```

**Warning: Flashing lights/colors in the gifs!**

Seems to work as expected with Visual Studio Code’s “[Live Preview](https://marketplace.visualstudio.com/items?itemName=ms-vscode.live-server)” extension.

> **VSCode Live Preview GIF**
>
> ![Recording 2026-10-16 210310](https://us1.discourse-cdn.com/flex001/uploads/mozilla/original/3X/a/2/a284ceecd0d8321b342f6b97298e955d8f3429d2.gif)

Does not work with Firefox’s “Responsive Design Mode”.

> **Responsive Design Mode GIF**
>
> ![responsive_design_mode](https://us1.discourse-cdn.com/flex001/uploads/mozilla/original/3X/3/f/3fdfef9bf13cb3f0440695ddabc3e2c0f0f4af49.gif)

My question is: Why doesn’t it work with Firefox? Is this a bug or an intended behavior?

I tested this with a couple of Chromium based browser and it worked with those.