Introduction to CSS

CSS is a styling language understood by practically every browser on the planet. Use it for the presentation aspects of your web applications.

Motivation

Separation of concerns is a powerful technique for managing complexity. Here are three things that are good to separate:

In web applications: Content=HTML, Presentation=CSS, Interactivity=JavaScript.

When ready for the gory details, see the CSS home page maintained by the W3C. When you’re ready for reference material, MDN has a great portal.

Demos First!

Start by seeing what you can do with CSS. This collection of 20 examples is pretty good. You should try out each of them before moving on.

The old CSS Zen Garden is still around. Take a look.

Tutorials

There are quite a few out there. HTML Dog has three good tutorials—one for beginners, one that is intermediate, and one that is advanced. You should do all three.

Technical Notes

Now that you have seen the demos and gone through a number of tutorials, let’s get technical.

(Also make a note of this fantastic reference site.)

Statements

A CSS style sheet consists of a sequence of statements. Each statement is either a ruleset or an at-rule.

Rulesets

A ruleset is a statement that tells the user agent (UA) how to render an element or elements.

ruleset.png

You can condense rulesets as follows:

Instead of...You can write...
h1 {font-weight: bold}
h2 {font-weight: bold}
h3 (font-weight: bold}
h1, h2, h3 {font-weight: bold}
h1 {color: green}
h1 {text-align: center}
h1 {
  color: green;
  text-align: center;
}

Properties

There are hundreds of propeties. In the early days of CSS1 there were just a few. CSS2 brought the count up to just over 100; CSS3 added hundreds more.

You might be interested in the W3C’s description of all the properties that were officially part of CSS back in 2010. Those are the most stable and widely supported.

Here is a sampling of most of the CSS3 properties:

Animationsanimation animation-delay animation-direction animation-duration animation-fill-mode animation-iteration-count animation-name animation-play-state animation-timing-function
Backgrounds and Bordersbackground background-attachment background-clip background-colo background-image background-origin background-position background-repeat background-size border border-bottom border-bottom-color border-bottom-left-radius border-bottom-right-radius border-bottom-style border-bottom-width border-color border-image border-image-outset border-image-repeat border-image-slice border-image-source border-image-width border-left border-left-color border-left-style border-left-width border-radius border-right border-right-color border-right-style border-right-width border-style border-top border-top-color border-top-left-radius border-top-right-radius border-top-style border-top-width border-width box-shadow
UI (Level 3)box-sizing caret-color cursor nav-down nav-left nav-right nav-up outline outline-color outline-offset outline-style outline-width resize text-overflow
UI (Level 4)appearance caret caret-animation caret-shape user-select
Box Alignmentalign-content align-items align-self justify-content justify-items justify-self
Cascading and Inheritance all
Colorcolor color-adjust opacity
Custom--*
Displaybox-suppress display
Exclusionswrap-flow wrap-through
Flexible Box Layout align-content align-items align-self flex flex-basis flex-direction flex-flow flex-grow flex-shrink flex-wrap justify-content order
Fonts font font-family font-feature-settings font-kerning font-language-override font-size font-size-adjust font-stretch font-style font-synthesis font-variant font-variant-alternates font-variant-caps font-variant-east-asian font-variant-ligatures font-variant-numeric font-variant-position font-weight
Fragmentation box-decoration-break break-after break-before break-inside orphans widows
Generated Content bookmark-label bookmark-level bookmark-state content quotes string-set
Generated Content for Paged Media bookmark-label bookmark-level bookmark-state footnote-display footnote-policy running string-set
Grid Layout grid grid-area grid-auto-columns grid-auto-flow grid-auto-rows grid-column grid-column-end grid-column-gap grid-column-start grid-gap grid-row grid-row-end grid-row-gap grid-row-start grid-template grid-template-areas grid-template-columns grid-template-rows
Image Values and Replaced Content image-orientation image-rendering image-resolution object-fit object-position
Inline Layout alignment-baseline baseline-shift dominant-baseline initial-letter initial-letter-align initial-letter-wrap vertical-align
Line Grid box-snap line-grid line-snap
Lists and Counters counter-increment counter-reset counter-set list-style list-style-image list-style-position list-style-type marker-side
Masking clip clip-path clip-rule mask mask-border mask-border-mode mask-border-outset mask-border-repeat mask-border-slice mask-border-source mask-border-width mask-clip mask-composite mask-image mask-mode mask-origin mask-position mask-repeat mask-size mask-type
Multicolumn Layout break-after break-before break-inside column-count column-fill column-gap column-rule column-rule-color column-rule-style column-rule-width column-span column-width columns
Overflow continue max-lines overflow overflow-x overflow-y
Page Floats clear float float-defer float-offset float-reference
Paged Mediapage size
Positioned Layout bottom left offset-after offset-before offset-end offset-start position right top z-index
Presentation Levelspresentation-level
Regions break-after break-before break-inside flow-from flow-into region-fragment
Round Display border-boundary polar-anchor polar-angle polar-distance polar-origin shape-inside
Ruby Layoutdisplay ruby-align ruby-merge ruby-position
Scroll Snap scroll-padding scroll-padding-block scroll-padding-block-end scroll-padding-block-start scroll-padding-bottom scroll-padding-inline scroll-padding-inline-end scroll-padding-inline-start scroll-padding-left scroll-padding-right scroll-padding-top scroll-snap-align scroll-snap-margin scroll-snap-margin-block scroll-snap-margin-block-end scroll-snap-margin-block-start scroll-snap-margin-bottom scroll-snap-margin-inline scroll-snap-margin-inline-end scroll-snap-margin-inline-start scroll-snap-margin-left scroll-snap-margin-right scroll-snap-margin-top scroll-snap-stop scroll-snap-type
Shapesshape-image-thresholdshape-marginshape-outside
Speech cue cue-after cue-before pause pause-after pause-before rest rest-after rest-before speak speak-as voice-balance voice-duration voice-family voice-pitch voice-range voice-rate voice-stress voice-volume
Tableborder-collapse border-spacing caption-side empty-cells table-layout
Template Layoutchains flow grid grid-template grid-template-areas grid-template-columns grid-template-rows
Text Decoration text-decoration text-decoration-color text-decoration-line text-decoration-skip text-decoration-style text-emphasis text-emphasis-color text-emphasis-position text-emphasis-style text-shadow text-underline-position
Text (Level 3) hanging-punctuation hyphens letter-spacing line-break overflow-wrap tab-size text-align text-align-all text-align-last text-indent text-justify text-transform white-space word-break word-spacing word-wrap
Text (Level 4) hyphenate-character hyphenate-limit-chars hyphenate-limit-last hyphenate-limit-lines hyphenate-limit-zone text-space-collapse text-space-trim text-spacing text-wrap white-space wrap-after wrap-before wrap-inside
Transformsbackface-visibility perspective perspective-origin transform transform-box transform-origin transform-style
Transitionstransition transition-delay transition-duration transition-property transition-timing-function
Will Changewill-change
Writing Modesdirection glyph-orientation-vertical text-combine-upright text-orientation unicode-bidi writing-mode
Basic Box Model clear display float height margin margin-bottom margin-left margin-right margin-top marquee-direction marquee-loop marquee-speed marquee-style max-height max-width min-height min-width overflow overflow-style overflow-x overflow-y padding padding-bottom padding-left padding-right padding-top rotation rotation-point visibility width
CSSOM Viewscroll-behavior
Compositing and Blendingbackground-blend-mode isolation mix-blend-mode
Filter Effectscolor-interpolation-filters filter flood-color flood-opacity lighting-color
Motion Pathmotion motion-offset motion-path motion-rotation offset offset-anchor offset-distance offset-path offset-position offset-rotation
SVG Markersmarker marker-end marker-knockout-left marker-knockout-right marker-mid marker-pattern marker-segment marker-start
SVG Strokesstroke stroke-alignment stroke-dashadjust stroke-dasharray stroke-dashcorner stroke-dashoffset stroke-linecap stroke-linejoin stroke-miterlimit stroke-opacity stroke-width

This set was taken from this list of all properties at the W3C site.

A good reference with lots of examples is HTML Dog’s CSS Property Reference.

Selectors

Easiest to learn these by example. See:

Values

Some properties have values that are special tokens, specific to that property (or a handful of properties), for example:

font-family: serif
font-variant: small-caps
text-align: center
text-decoration: underline
margin-left: auto
border-left-width: medium
list-style-position: inside
list-style-type: square
white-space: pre
cursor: pointer
overflow: scroll
visibility: hidden
clear: both
display: table-row
position: absolute

The values inherit, initial, and unset apply everywhere.

Sometimes the property value comes from a specific type. Here are some of the types in CSS, together with example values:

TypeExample values
length
percentage
number
integer
string
uri
color
resolution
angle
time
frequency
ratio

And, you guessed it, you should see the HTML Dog CSS Value Reference.

To find which values go with which properties, you should check the official docs for that module. However, you can get a good idea of the value assignments by checking out this list and this list, which both give mappings for a subset of all properties.

At-rules

An at-rule is kind of like an instruction to the CSS parser. The common at-rules are:

Inheritance

<html>
  <head>
    <title>Another Example</title>
    <style>
      body {color: green; background: white;}
      h1 {color: navy}
    </style>
  </head>
  <body>
    <h1>Test</h1>
    <p>Here is an <strong>example</strong> of</p>
    <ul>
      <li>Style sheets</li>
      <li>Inheritance of properties</li>
      <li>Overriding</li>
    </ul>
  </body>
</html>

cssinheritance.gif

Exercise: You can give a property the explicit value of inherit which means the corresponding element gets same value for that property as its parent. This seems to be default behavior. Look up and describe when you would ever want to do this.

The Cascade

TODO

Specificity

TODO

Connecting a stylesheet to a document

Quite a few ways:

  1. In an XML <?xml-stylesheet?> processing instruction.
  2. In an HTML <link> element.
  3. In an HTML <style> element. Use only for hacks, never in serious programming.
  4. As a style attribute in any HTML element that allows it. This is called an inline style. Don’t use.

xml-stylesheet Processing Instruction

Example (from Harold and Means’ text):

<?xml-stylesheet href="recipe.css" media="screen"
           alternate="no" title="For Web Browsers" charset="US-ASCII"?>
<?xml-stylesheet href="printable_recipe.css" media="print"
           alternate="no" title="For Printing" charset="ISO-8859-1"?>
<?xml-stylesheet href="big_recipe.css" media="projection"
           alternate="no" title="For presentations" charset="UTF-8"?>
<?xml-stylesheet href="tty_recipe.css" media="tty"
           alternate="no" title="For Lynx" charset="US-ASCII"?>
<?xml-stylesheet href="small_recipe.css" media="handheld"
           alternate="no" title="For Palm Pilots" charset="US-ASCII"?>

The possible values for media are: screen, tty, tv, projection, handheld, print, braille, aural, all.

The <link> element

This is the most common way and almost always the best. Example:

<html>
  <head>
    <title>Another Example</title>
    <link rel="stylesheet" href="plain.css" media="screen">
  </head>
  <body>
    <p>Hello</p>
  </body>
</html>

The <style> element

Example:

<html>
  <head>
    <title>Another Example</title>
    <style>
      body {padding: 2em; background: white;}
      p {font: 36pt serif}
    </style>
  </head>
  <body>
    <p>Hello</p>
  </body>
</html>
You probably should not be using these.

Use only in tiny web page hacks and prototypes. You should be cleanly separating your content and presentation.

Inline styles

Example:

<h1 style="color:blue; text-align:right">Hello</h1>
<div style="position:absolute; z-index:1; left:20px; top:160px; width:150px">
Do not use these.

They are like gotos and evals.

Bad programmer. Bad.

Examples

Here are some snippets of moderate interest, nothing fancy:

  h1 {
    font-weight: bold;
    font-size: 12pt;
    line-height: 14pt;
    font-family: Helvetica;
    font-variant: normal;
    font-style: normal;
    font-stretch: normal;
    font-size-adjust: none
  }

  h1 { font: bold 12pt/14pt Helvetica }

  ol li {list-style: upper-alpha}
  ol ol li {list-style: upper-roman}
  ol ol ol li {list-style: lower-alpha}
  ol ol ol ol li {list-style: decimal}

These are non-examples, showing common mistakes:

  p   { font-vendor: any }     /* Invalid prop.: font-vendor */
  h1  { font-style: 12pt }     /* Invalid value for prop: 12pt */
  h1  { rotation: 70minutes }  /* 70minutes is not a valid value */
  img { float: left here }     /* "here" is not a value of "float" */
  img { background: "red" }    /* keywords cannot be quoted in CSS2 */
  img { border-width: 3 }      /* a unit must be specified for length values */
  h3, h4 & h5 {color: red }    /* & is an invalid token- WHOLE LINE must be ignored */

Box Model

TODO

CSS Transforms

TODO

Here is a talk and demo by Ana Tudor showing off what you can do with CSS and no JavaScript:

You should also browse her collection of code pens.

Media Queries and Responsive Design

Ideally you should make applications that work correctly on all kinds of different media—PCs, handhelds, phones, tablets, speech output devices, print, etc. All differ widely in resolution, color depth, bandwidth, etc.

You can learn the basics in the Responsive Web Design section of Google’s Web Fundamentals course.