What is CSS?

December 27th, 2019


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What is CSS?

CSS stands for Cascading Stylesheets. HTML defines the contents in web pages while CSS describes how contents are displayed on the browser.

What is a Stylesheet?

A stylesheet is a collection of style rules declared to have effect on the contents of a web page

The term 'Cascading'

Stylesheets come from different sources which may have conflicting styles. The work of the cascade is to determine which style takes priority.
The different sources are:

  • User-agent stylesheets
    The user agent is the tool (usually a web browser) which is used in opening a webpage. Every browser has default styles for contents of a webpage which are applied if there are no declarations for such contents. For this reason, browsers display in webpages differently if there is not constant declaration used for the pages.
  • Author stylesheets
    This contains style rules declared by the creator of a website. When browsers try to open a website, they search for the styles of the elements from this sheet before defaulting to their own rules.
  • User stylesheets
    The user viewing a website can also override the styles in the author or user-agent stylesheet by declaring desired styles for the page. However, this does not affect the website on the server but only on the user's computer.

The cascade filters all rules declared on the different sources for each elements and determines which would take priority.

Putting these terms together,
CSS is used to define how elements on a page are displayed. Its duty is to add beauty to a website by creating layouts for contents, adding colors, creating animations, etc which is displayed on different websites.

Different tags have their own properties. For example, text elements like headers and paragraphs have a font-size property which can be used to specify their sizes. Obviously this property cannot be used for images.

Not all tags have the same properties. You'd have to know the properties an element possesses before you can describe the display. If the property declared isn't possessed by the element, CSS will ignore it.

Note that there is a range of values which can be set for any property.

Using CSS With HTML

CSS can be used with HTML in three ways:

  • Inline Styling
  • Internal styling
  • External styling

Inline Styling

In this method, the style is declared in the same html file. style is used as an attribute (property) for tags for describing how the elements are displayed with property and value of the element like this;

<tag style="property: value; anotherProperty: value">
<!-- For a paragraph, we could have -->
<p style="color: red; font-size: 35px; text-align: center;">
  I am a paragraph

px is a type of measurement. There's also em, rem and so on. Check out this article for more information - Point Measurements in CSS

Internal Styling.

In this method, the style is also declared in the same html file but the declarations have their own block created with the style tag. The syntax for declaring style blocks is;

  element {
    property: value;
    anotherproperty: value;

  anotherElement {
    property: value;
    anotherproperty: value;

This block is usually declared in the <head> section of a document but it can be declared even in the <body>. It targets an element and sets a property with a desired value (from the range of values available for that property). This is followed by a semi-colon (;) which terminates the line. If this character is missed no a line, CSS would not interpret it.

By translating our inline style above to this method, we would have;

  p {
    color: red;
    font-size: 35px;
    text-align: center;

Note that the spaces do not mean anything to the browser. We could have everything on one line with semicolons stated appropriately like this;

  p {
    color: red;
    font-size: 35px;
    text-align: center;

but readability would be difficult. This is the same reason why HTML elements are arranged spaciously.

External Styling

Unlike the above methods, this involves using the css declarations in another file which is with an extension of .css. That file is then referenced by the html document for the styles to take effect. This file would be similar to the internal styling syntax above but without the style tag.

In our .css file, say style.css, we could have the internal styling above converted to;

p {
  color: red;
  font-size: 35px;
  text-align: center;

To reference the css file in the html, the <link> tag which contains a void element would be added to the head tag.

<!DOCTYPE html>
    <link rel="stylesheet" href="path/to/style.css" />

The rel attribute shows the relationship between the linked and the current document. The value used is stylesheet, which means the file is a css file.

href is an attirbute which specifies the location of the file which is referenced. Relative or absolute links can be used as long as you can reach the file.

However, it is good practice to have stylings declared in another file. This aids readability (as there is a seperation between html codes and css codes) thereby providing easy in code editing and contributions.


Comments help you locate certain areas of your codes easily. You have a 10-line code and feel its unnecessary to put any comments, but what if you like up to 500 lines of code or more. Comments can help you remember what elements you're styling or the purpose of your styles in some part of the CSS codes. They are ignored by the browser.

Syntax for CSS comments is;

/* single-line comments */

The first is for single line comments while the second is for multi lines. In short, anything found between /* and */ is a comment.

In the above codes, we selected the elements we wanted to style by their tag names. This means that every element on the page with that tag will be affected by the style declarations. On most occassions, this is not our intention. Sometimes, a developer may just want to select some paragraphs or some images to style. This is possible. Check out this article to know how - Selector Methods in CSS

There's more to CSS which we would be exploring in future articles. Stay tuned!

A building construction example with HTML, CSS and Javascript

This illustration for building a house was from Intro to Web Development in Frontend Masters. I also used the in an Introduction to web development presentation I made and in other articles on this website

HTML consist of the blocks, wood, roof, basically the contents of the building. Without CSS, the blocks of the building are just placed on themselves as you keep adding.

CSS determines the different styles which are applied to the contents to make the building firm and attractive. i.e CSS accurately determines the position and width of each blocks, the color of the roof, the particular way a content should be displayed and so much more. With HTML and CSS, you already have a building. Javascript may not be necessary.

Javascript comes in when you need interaction in your building. What does interaction mean? When the user opens the door with a key, what should the door do? Open right? Interaction. The HTML and CSS just defines the structure of the building, but there's no response to action. Javascript enables the actions.

Learn more on HTML in this article - What is HTML? and Javascript in this article - What is Javascript?.

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