---
title: Introduction to Basic CSS
block: Basic CSS
superBlock: Responsive Web Design
---
## Introduction to Basic CSS

Cascading Style Sheets (CSS) tell the browser how to display the text and other content that you write in HTML.

Note that CSS is case-sensitive so be careful with your capitalization.

CSS has been adopted by all major browsers and allows you to control:

- color
- fonts
- positioning
- spacing
- sizing
- decorations
- transitions
  
There are three main ways to apply CSS styling. You can apply inline styles directly to HTML elements with the `style` attribute. Alternatively, you can place CSS rules within `style` tags in an HTML document. Finally, you can write CSS rules in an external style sheet, then reference that file in the HTML document. Even though the first two options have their use cases, most developers prefer external style sheets because they keep the styles separate from the HTML elements. This improves the readability and reusability of your code.

The idea behind CSS is that you can use a selector to target an HTML element in the DOM (Document Object Model) and then apply a variety of attributes to that element to change the way it is displayed on the page.

In this section, you'll see how adding CSS styles to the elements of your CatPhotoApp can change it from simple text to something more.

<div role="alert" class="fade alert alert-info show flash-message-enter-active">
    <p>
        Note: Some browser extensions may interfere with elements on the page. If the tests fail, try disabling your extensions for the most reliable experience.
    </p>
</div>
