Introduction to CSS
Motivation
Separation of concerns is a powerful technique for managing complexity. Here are three things that are good to separate:
- Content. What is your data and how is structured? Think canvases, headings, menus, footers, articles, sections, paragraphs, lists, tables, videos, interactive controls (buttons, sliders, text input fields).
- Presentation. How does your data appear? Think sizes, fonts, spacing, aural cues, colors, borders, margins, shadows, positioning (relative, absolute, fixed, grid, flex, float, 2-D and 3-D transforms), transitioning and animating.
- Interactivity What events occur and what should be done in response? (This is a good question for now, but keep and open mind about what interactivity really is, or should be, or could be.)
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.
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 properties. 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:
| Animations | animation animation-delay animation-direction animation-duration animation-fill-mode animation-iteration-count animation-name animation-play-state animation-timing-function |
|---|---|
| Backgrounds and Borders | background 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 Alignment | align-content align-items align-self justify-content justify-items justify-self |
| Cascading and Inheritance | all |
| Color | color color-adjust opacity |
| Custom | --* |
| Display | box-suppress display |
| Exclusions | wrap-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 Media | page size |
| Positioned Layout | bottom left offset-after offset-before offset-end offset-start position right top z-index |
| Presentation Levels | presentation-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 Layout | display 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 |
| Shapes | shape-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 |
| Table | border-collapse border-spacing caption-side empty-cells table-layout |
| Template Layout | chains 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 |
| Transforms | backface-visibility perspective perspective-origin transform transform-box transform-origin transform-style |
| Transitions | transition transition-delay transition-duration transition-property transition-timing-function |
| Will Change | will-change |
| Writing Modes | direction 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 View | scroll-behavior |
| Compositing and Blending | background-blend-mode isolation mix-blend-mode |
| Filter Effects | color-interpolation-filters filter flood-color flood-opacity lighting-color |
| Motion Path | motion motion-offset motion-path motion-rotation offset offset-anchor offset-distance offset-path offset-position offset-rotation |
| SVG Markers | marker marker-end marker-knockout-left marker-knockout-right marker-mid marker-pattern marker-segment marker-start |
| SVG Strokes | stroke 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:
- The official W3C Recommendation, with the selector definitions, syntax, and lots of examples.
- The summary in the W3C spec.
- HTML Dog’s CSS Selectors Reference, with lots of examples.
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:
| Type | Example 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.
Speaking of color, here’s something you should watch if you think that red, yellow, and blue are primary colors (SPOILER: THEY ARE NOT):
At-rules
An at-rule is kind of like an instruction to the CSS parser. The common at-rules are:
@charset@import@media@page@font-face@namespace
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>
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
TODOSpecificity
TODOConnecting a stylesheet to a document
Quite a few ways:
- In an HTML
<link>element. - In an HTML
<style>element. Use only for hacks, never in serious programming. - As a
styleattribute in any HTML element that allows it. This is called an inline style. Don’t use.
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>
Possible values for media include: screen, tty, tv, projection, handheld, print, braille, aural, all.
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
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.