1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
|
---
language: css
contributors:
- ["Mohammad Valipour", "https://github.com/mvalipour"]
- ["Marco Scannadinari", "https://github.com/marcoms"]
- ["Geoffrey Liu", "https://github.com/g-liu"]
- ["Connor Shea", "https://github.com/connorshea"]
- ["Deepanshu Utkarsh", "https://github.com/duci9y"]
filename: learncss.css
---
In the early days of the web there were no visual elements, just pure text. But with further development of web browsers, fully visual web pages also became common.
CSS helps maintain separation between the content (HTML) and the look-and-feel of a web page.
CSS lets you target different elements on an HTML page and assign different visual properties to them.
This guide has been written for CSS 2, though CSS 3 is fast becoming popular.
**NOTE:** Because CSS produces visual results, in order to learn it, you need try everything in a CSS playground like [dabblet](http://dabblet.com/).
The main focus of this article is on the syntax and some general tips.
```css
/* comments appear inside slash-asterisk, just like this line!
there are no "one-line comments"; this is the only comment style */
/* ####################
## SELECTORS
#################### */
/* the selector is used to target an element on a page.
selector { property: value; /* more properties...*/ }
/*
Here is an example element:
<div class='class1 class2' id='anID' attr='value' otherAttr='en-us foo bar' />
*/
/* You can target it using one of its CSS classes */
.class1 { }
/* or both classes! */
.class1.class2 { }
/* or its name */
div { }
/* or its id */
#anID { }
/* or using the fact that it has an attribute! */
[attr] { font-size:smaller; }
/* or that the attribute has a specific value */
[attr='value'] { font-size:smaller; }
/* starts with a value (CSS 3) */
[attr^='val'] { font-size:smaller; }
/* or ends with a value (CSS 3) */
[attr$='ue'] { font-size:smaller; }
/* or contains a value in a space-separated list */
[otherAttr~='foo'] { }
[otherAttr~='bar'] { }
/* or contains a value in a dash-separated list, ie, "-" (U+002D) */
[otherAttr|='en'] { font-size:smaller; }
/* You can concatenate different selectors to create a narrower selector. Don't
put spaces between them. */
div.some-class[attr$='ue'] { }
/* You can select an element which is a child of another element */
div.some-parent > .class-name { }
/* or a descendant of another element. Children are the direct descendants of
their parent element, only one level down the tree. Descendants can be any
level down the tree. */
div.some-parent .class-name { }
/* Warning: the same selector without a space has another meaning.
Can you guess what? */
div.some-parent.class-name { }
/* You may also select an element based on its adjacent sibling */
.i-am-just-before + .this-element { }
/* or any sibling preceding it */
.i-am-any-element-before ~ .this-element { }
/* There are some selectors called pseudo classes that can be used to select an
element when it is in a particular state */
/* for example, when the cursor hovers over an element */
selector:hover { }
/* or a link has been visited */
selector:visited { }
/* or hasn't been visited */
selected:link { }
/* or an element in focus */
selected:focus { }
/* At appropriate places, an asterisk may be used as a wildcard to select every
element */
* { } /* all elements */
.parent * { } /* all descendants */
.parent > * { } /* all children */
/* ####################
## PROPERTIES
#################### */
selector {
/* Units of length can be absolute or relative. */
/* Relative units */
width: 50%; /* percentage of parent element width */
font-size: 2em; /* multiples of element's original font-size */
font-size: 2rem; /* or the root element's font-size */
font-size: 2vw; /* multiples of 1% of the viewport's width (CSS 3) */
font-size: 2vh; /* or its height */
font-size: 2vmin; /* whichever of a vh or a vw is smaller */
font-size: 2vmax; /* or greater */
/* Absolute units */
width: 200px; /* pixels */
font-size: 20pt; /* points */
width: 5cm; /* centimeters */
min-width: 50mm; /* millimeters */
max-width: 5in; /* inches */
/* Colors */
color: #F6E; /* short hex format */
color: #FF66EE; /* long hex format */
color: tomato; /* a named color */
color: rgb(255, 255, 255); /* as rgb values */
color: rgb(10%, 20%, 50%); /* as rgb percentages */
color: rgba(255, 0, 0, 0.3); /* as rgba values (CSS 3) Note: 0 < a < 1 */
color: transparent; /* equivalent to setting the alpha to 0 */
color: hsl(0, 100%, 50%); /* as hsl percentages (CSS 3) */
color: hsla(0, 100%, 50%, 0.3); /* as hsla percentages with alpha */
/* Images as backgrounds of elements */
background-image: url(/img-path/img.jpg); /* quotes inside url() optional */
/* Fonts */
font-family: Arial;
/* if the font family name has a space, it must be quoted */
font-family: "Courier New";
/* if the first one is not found, the browser uses the next, and so on */
font-family: "Courier New", Trebuchet, Arial, sans-serif;
}
```
## Usage
Save a CSS stylesheet with the extension `.css`.
```xml
<!-- You need to include the css file in your page's <head>. This is the
recommended method. Refer to http://stackoverflow.com/questions/8284365 -->
<link rel='stylesheet' type='text/css' href='path/to/style.css' />
<!-- You can also include some CSS inline in your markup. -->
<style>
a { color: purple; }
</style>
<!-- Or directly set CSS properties on the element. -->
<div style="border: 1px solid red;">
</div>
```
## Precedence or Cascade
An element may be targeted by multiple selectors and may have a property set on it in more than once. In these cases, one of the rules takes precedence over others. Generally, a rule in a more specific selector take precedence over a less specific one, and a rule occuring later in the stylesheet overwrites a previous one.
This process is called cascading, hence the name Cascading Style Sheets.
Given the following CSS:
```css
/* A */
p.class1[attr='value']
/* B */
p.class1 { }
/* C */
p.class2 { }
/* D */
p { }
/* E */
p { property: value !important; }
```
and the following markup:
```xml
<p style='/*F*/ property:value;' class='class1 class2' attr='value' />
```
The precedence of style is as follows. Remember, the precedence is for each **property**, not for the entire block.
* `E` has the highest precedence because of the keyword `!important`. It is recommended that you avoid its usage.
* `F` is next, because it is an inline style.
* `A` is next, because it is more "specific" than anything else. It has 3 specifiers: The name of the element `p`, its class `class1`, an attribute `attr='value'`.
* `C` is next, even though it has the same specificity as `B`. This is because it appears after `B`.
* `B` is next.
* `D` is the last one.
## Compatibility
Most of the features in CSS 2 (and many in CSS 3) are available across all browsers and devices. But it's always good practice to check before using a new feature.
## Resources
* To run a quick compatibility check, [CanIUse](http://caniuse.com).
* CSS Playground [Dabblet](http://dabblet.com/).
* [Mozilla Developer Network's CSS documentation](https://developer.mozilla.org/en-US/docs/Web/CSS)
* [Codrops' CSS Reference](http://tympanus.net/codrops/css_reference/)
## Further Reading
* [Understanding Style Precedence in CSS: Specificity, Inheritance, and the Cascade](http://www.vanseodesign.com/css/css-specificity-inheritance-cascaade/)
* [Selecting elements using attributes](https://css-tricks.com/almanac/selectors/a/attribute/)
* [QuirksMode CSS](http://www.quirksmode.org/css/)
* [Z-Index - The stacking context](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context)
* [SASS](http://sass-lang.com/) and [LESS](http://lesscss.org/) for CSS pre-processing
* [CSS-Tricks](https://css-tricks.com)
|
