CSS：Tutorial one

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发布时间：2019-06-18

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1、Three Ways to Insert CSS

External style sheet

Internal style sheet

Inline style

External Style Sheet

With an external style sheet, you can change the look of an entire website by changing just one file!

Each page must include a reference to the external style sheet file inside the <link> element. The <link> element goes inside the <head> section:

Internal Style Sheet

An internal style sheet may be used if one single page has a unique style.

Internal styles are defined within the <style> element, inside the <head> section of an HTML page:

Inline Styles

An inline style may be used to apply a unique style for a single element.

To use inline styles, add the style attribute to the relevant element. The style attribute can contain any CSS property.

The example below shows how to change the color and the left margin of a <h1> element:

This is a heading

2、CSS Colors

Colors are specified using predefined color names, or RGB, HEX, HSL, RGBA, HSLA values.

backgroud color 、 text color 、border color

Color Values

In HTML, colors can also be specified using RGB values, HEX values, HSL values, RGBA values, and HSLA values:

Same as color name "Tomato":

rgb(255, 99, 71)#ff6347hsl(9, 100%, 64%)

Same as color name "Tomato", but 50% transparent:

rgba(255, 99, 71, 0.5)hsla(9, 100%, 64%, 0.5)

3、CSS Backgrounds

The CSS background properties are used to define the background effects for elements.

CSS background properties:

background-color

background-image

background-repeat

background-attachment

background-position

Background Image

The background-image property specifies an image to use as the background of an element.

By default, the image is repeated so it covers the entire element.

The background image for a page can be set like this:

body {
      background-image: url("paper.gif");}

By default, the background-image property repeats an image both horizontally and vertically.

body {
      background-image: url("gradient_bg.png");  background-repeat: repeat-x;}

Background Image - Fixed position

To specify that the background image should be fixed (will not scroll with the rest of the page), use the background-attachment property:

body {
      background-image: url("img_tree.png");  background-repeat: no-repeat;  background-position: right top;  background-attachment: fixed;}

Background - Shorthand property

To shorten the code, it is also possible to specify all the background properties in one single property. This is called a shorthand property.

The shorthand property for background is background:

body {
      background: #ffffff url("img_tree.png") no-repeat right top;}

4、CSS Borders

Border Style

The border-style property specifies what kind of border to display.

The following values are allowed:

dotted - Defines a dotted border

dashed - Defines a dashed border

solid - Defines a solid border

double - Defines a double border

groove - Defines a 3D grooved border. The effect depends on the border-color value

ridge - Defines a 3D ridged border. The effect depends on the border-color value

inset - Defines a 3D inset border. The effect depends on the border-color value

outset - Defines a 3D outset border. The effect depends on the border-color value

none - Defines no border

hidden - Defines a hidden border

The border-style property can have from one to four values (for the top border, right border, bottom border, and the left border).

Border Width

The border-width property specifies the width of the four borders.

The width can be set as a specific size (in px, pt, cm, em, etc) or by using one of the three pre-defined values: thin, medium, or thick.

The border-width property can have from one to four values (for the top border, right border, bottom border, and the left border).

Border - Shorthand Property

You can also specify all the individual border properties for just one side:

p {
      border-left: 6px solid red;  background-color: lightgrey;}

Rounded Borders

The border-radius property is used to add rounded borders to an element:

p {
      border: 2px solid red;  border-radius: 5px;}

5、CSS Margins

he CSS margin properties are used to create space around elements, outside of any defined borders.

With CSS, you have full control over the margins. There are properties for setting the margin for each side of an element (top, right, bottom, and left).

If the margin property has three values:

margin: 25px 50px 75px;
- top margin is 25px
- right and left margins are 50px
- bottom margin is 75px

If the margin property has two values:

margin: 25px 50px;
- top and bottom margins are 25px
- right and left margins are 50px

The auto Value

You can set the margin property to auto to horizontally center the element within its container.

The element will then take up the specified width, and the remaining space will be split equally between the left and right margins:

div {
        width: 300px;  margin: auto;  border: 1px solid red;}

The inherit Value

This example lets the left margin of the <p class="ex1"> element be inherited from the parent element (<div>):

Use of the inherit value

Let the left margin be inherited from the parent element:

This paragraph has an inherited left margin (from the div element).

Margin Collapse

Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins.

This does not happen on left and right margins! Only top and bottom margins!

Look at the following example:

In this example the h1 element has a bottom margin of 50px and the h2 element has a top margin of 20px. Then, the vertical margin between h1 and h2 should have been 70px (50px + 20px). However, due to margin collapse, the actual margin ends up being 50px.

Heading 1

Heading 2

All HTML elements can be considered as boxes. In CSS, the term "box model" is used when talking about design and layout.

The CSS box model is essentially a box that wraps around every HTML element. It consists of: margins, borders, padding, and the actual content.

similar padding

6、CSS Box Model

All HTML elements can be considered as boxes. In CSS, the term "box model" is used when talking about design and layout.

The CSS box model is essentially a box that wraps around every HTML element. It consists of: margins, borders, padding, and the actual content.

Explanation of the different parts:

Content - The content of the box, where text and images appear

Padding - Clears an area around the content. The padding is transparent

Border - A border that goes around the padding and content

Margin - Clears an area outside the border. The margin is transparent

7、CSS Outline

An outline is a line that is drawn around elements, OUTSIDE the borders, to make the element "stand out".

CSS has the following outline properties:

outline-style

outline-color

outline-width

outline-offset

outlin

Note: Outline differs from borders! Unlike border, the outline is drawn outside the element's border, and may overlap other content. Also, the outline is NOT a part of the element's dimensions; the element's total width and height is not affected by the width of the outline.

The outline-style property specifies the style of the outline, and can have one of the following values:

dotted - Defines a dotted outline

dashed - Defines a dashed outline

solid - Defines a solid outline

double - Defines a double outline

groove - Defines a 3D grooved outline

ridge - Defines a 3D ridged outline

inset - Defines a 3D inset outline

outset - Defines a 3D outset outline

none - Defines no outline

hidden - Defines a hidden outline

Outline - Shorthand property

The outline property is a shorthand property for setting the following individual outline properties:

outline-width

outline-style (required)

outline-color

The outline property is specified as one, two, or three values from the list above. The order of the values does not matter.

p.ex1 {
    outline: dashed;}p.ex2 {
    outline: dotted red;}p.ex3 {
    outline: 5px solid yellow;}p.ex4 {
    outline: thick ridge pink;}

Outline Offset

The outline-offset property adds space between an outline and the edge/border of an element. The space between an element and its outline is transparent.

The following example specifies an outline 15px outside the border edge: