M1 Introduction to CSS

CSS (Cascading Style Sheets)

Notice that we haven't said a word about how things look yet. This is because that is what CSS is for!

CSS stands for Cascading Style Sheets, and it defines the appearance of your page. CSS is composed of rules, and each rule consists of a selector and one or more declarations. Let's see an example:

header {
	color: blue;
	font-family: Verdana, sans-serif;
}

The above CSS rule does three things:

• It selects all <header> elements
• It declares the property color (i.e. the text color) of the selected elements to be blue
• It declares the property font-family (i.e. the font "face") of the selected elements to be Verdana, or alternatively any sans-serif font if Verdana is not available

All CSS rules follow this general format. The following sections will discuss more detail on CSS selectors and declarations.

Selectors

Watch on DOCTYPE

CSS Selectors, Part 1

In the previous section's example, the most basic CSS selector was used: selecting by the tag name. To select by the class name, use a period, like so:

.info {
	font-weight: bold;
}

Watch on DOCTYPE

CSS Selectors, Part 2

A third common way to select an element is by its ID. To select by ID, use a number sign, like so:

#medal_table {
	border-collapse: collapse;
}

Selectors can be combined. For example, to select all hyperlinks in any paragraph having the class "alert", you could do:

p.alert a {
	font-style: italic;
}

The p.alert selects all <p> tags with the class name "alert". The space tells CSS to look for all descendants of such paragraphs. Finally, the a selects all <a> elements in the resulting set.

It is also very common to use a right-angle bracket to select a child rather than a descendant, the distinction being that descendants could be nested at any depth: child, grandchild, and so on, while a child must be at the next level down in the DOM. For example:

ul.fruits > li {
	text-decoration: underline;
}

Some common CSS selectors are listed below. For a complete list of CSS selectors, see section 4.2 of the CSS Specification.

• foo selects an element with tag name foo
• .foo selects an element with the class name foo
• #foo selects an element with the ID foo
• [foo="bar"] selects an element with the attribute foo whose value is bar
• foo bar selects an element matching bar that is a descendant of an element matching foo
• foo > bar selects an element matching car that is a child of an element matching foo
• foo + bar selects an element matching bar that is the next sibling of an element matching foo

Pseudo-Classes

Elements can also be selected based on their current state using pseudo-classes. Pseudo-classes always begin with a colon. For example, this is how you could change the color of all visited links on your web page:

a:visited {
	color: #99FF99; /* this example uses a hexadecimal RGB definition for the color, which allows you to
		mix over 16 million colors */
}

Some common pseudo-classes are listed below. For a complete list, see section 4.2 of the CSS Specification.

• foo:first-child selects an element matching foo that is the first child of its parent (i.e., any foo that doesn't have older siblings)
• foo:last-child selects an element matching foo that is the last child of its parent
• foo:empty selects any element matching foo that has no children
• Pseudo-Classes for Form Elements:
  • input:checked selects all checkboxes and radio buttons that are checked
• Pseudo-Classes for Hyperlink Elements:
  • a:link selects all hyperlinks that have not been visited yet
  • a:visited selects all hyperlinks that have been visited
• Pseudo-Classes for Interactive Elements (like buttons, hyperlinks, etc):
  • foo:active selects elements matching foo that are being pressed on
  • foo:hover selects elements matching foo that are being hovered over
  • foo:focus selects elements matching foo that are selected (no pun intended)

Properties

Selectors are used to select elements that need to have styles applied to them, and properties are used to actually define those styles. Throughout this section, you've seen properties including color, font-family, font-style, and text-decoration. There are dozens more properties you can define, some of which are listed below.

For a complete list of CSS properties, see section 4.1 of the CSS Specification.

• background-image sets the background image of an element. Usage: background-image: url(path/to/image.png);
• background-color sets the background color of an element (note: the image takes precedence over the color; the color will be displayed if the image can't be found)
• For Tables:
  • border-collapse removes the spacing between table cells. Usage: border-collapse: collapse;
  • Note: You can select td elements that are grandchildren of a table element, and then specify border properties on the td's, like this: #myTable > tr > td { border: 1px solid black; }
• border defines the border around an element. Usage: border: 1px dashed red;
• border-radius lets you have rounded corners on your elements
• float makes the text of a document flow around the element. Usage: float: left; (or right)
  • clear makes an element stop wrapping around floats and instead move down to where the floats end. Usage: clear: both;
  • Note: If you specify multiple object in a for as floating, they will appear next to each other horizontally.
• cursor lets you change the type of cursor that appears then your element is in focus. Example usage: cursor: pointer;
• font lets you specify the font-size, line-height, and font-family in one fell swoop. Usage: font: 14px/18px Verdana, Arial, sans-serif;
• list-style-type specifies the type of number or bullet used on <ol>'s and <ul>'s
• text-align lets you justify text on the left, center, or right
• display lets us specify an element as inline or block (see below), and it also enables us to "turn off" an element from being displayed on a page. Example usage: display: none;

Units

Some CSS properties require that you specify numerical values for things (font-size, line-height, border-width, and so on). CSS supports a number of units, the most common of which include:

• px, which defines a distance in pixels (relative to screen resolution).
• em, which defines a distance relative to font size. (Note: an em is the width of the capital letter M. No, I'm not kidding.)
• rem, which defines a distance relative to the font size of the root element. Compare to em, which defines a distance relative to the font size of the current element (which might have a different font size than the document root).
• pt, which is a traditional print unit equal to 1/72 of an inch (should appear the same size on all screens regardless of resolution).
• %, which defines a distance relative to that property of its parent element. For the most part, you'll only use % when working with the Box Model (see below).

The Box Model

For the most part, all elements fall into one of two categories:

• Inline (Examples: a, span, strong)
• Block (Examples: p, div, section)

The properties we've seen thus far apply mostly to inline elements. As it turns out, CSS has an additional set of properties that describe block elements. These properties compose the box model.

While inline elements flow with the text, block elements stand on their own. You can define the space between the block element and the next closest element on the page using margin, the block element's border using border, and the space between the block element's border and its content using padding:

For example, you could make a padded box that is centered on the page using this CSS:

div.box {
	width: 500px;
	margin: 0 auto; /* when two "arguments" are passed to margin or padding, the first defines top
		and bottom margin/padding and the second defines left and right margin/padding */
	padding: 25px; /* when only one "argument" is passed to margin or padding, it defines the
		margin/padding on all four sides */
	border: 1px solid black; /* the 1px defines the border-width; the solid defines the border-style;
		the black defines the border-color */
}

Here is what the resulting box looks like:

Note that the width defines only the content width. To compute the actual "width" of the element, you need to add the left and right padding border, and margin.

This is only scraping the surface of the CSS box model. All advanced web sites utilize the box model to its full extent to produce the complex layouts you have come to expect on the internet. For more information on the CSS box model, read the W3C specification.

Typography

Traditionally, Web developers were limited to a small subset of fonts that were commonly installed on all computers:

• Andale Mono
• Arial
• Arial Black
• Comic Sans MS
• Courier New
• Georgia
• Impact
• Times New Roman
• Trebuchet MS
• Verdana
• Webdings

That was it. Incredibly limiting.

However, fonts other than these are used widely on the Web. How? CSS has evolved to the point where it now allows you to upload a font to your server, and use that font on your site, or even include CSS from another server that adds the fonts you want. One excellent resource for a wide variety of fast, appropriately-licensed fonts is Google Fonts.