# CSS
Cascading Style Sheets (CSS) is a language used to describe the HML page structure. All HML components have default styles. You can customize styles for these components using CSS to design various pages.
## Size Unit
1. Logical px set by <length>:
1. The default logical screen width is 720 px (for details, see the "window" section in [config.json](js-framework-js-tag.md)). Your page will be scaled to fit the actual width of the screen. For example, on a screen with the actual width of 1440 physical px, 100 px is displayed on 200 physical px, with all sizes doubled from 720 px to 1440 px.
2. If you set autoDesignWidth to true (for details, see the "window" section in [config.json](js-framework-js-tag.md)), the logical px are scaled based on the screen density. For example, if the screen density is 3x, 100 px will be rendered on 300 physical px. This approach is recommended when your application needs to adapt to multiple devices.
2. Percentage set by <percentage>: The component size is represented by its percentage of the parent component size. For example, if the width <percentage> of a component is set to 50%, the width of the component is half of its parent component's width.
## Style Import
CSS files can be imported using the @import statement. This facilitates module management and code reuse.
## Style Declaration
The .css file with the same name as the .hml file in each page directory describes the styles of components on the HML page, determining how the components will be displayed.
1. Internal style: The style and class attributes can be used to specify the component style. Example:
```
Hello World
```
```
/* index.css */
.container {
justify-content: center;
}
```
2. External style files: You need to import the files. For example, create a style.css file in the common directory and import the file at the beginning of index.css.
```
/* style.css */
.title {
font-size: 50px;
}
```
```
/* index.css */
@import '../../common/style.css';
.container {
justify-content: center;
}
```
## Selectors
A CSS selector is used to select elements for which styles need to be added to. The following table lists the supported selectors.
| Selector | Example | Description |
| -------- | -------- | -------- |
| .class | .container | Selects all components whose class is container. |
| \#id | \#titleId | Selects all components whose id is titleId. |
| tag | text | Selects all <text> components. |
| , | .title, .content | Selects all components whose class is title or content. |
| \#id .class tag | \#containerId .content text | Selects all grandchild <text> components whose grandparent components are identified as containerId and whose parent components are of the content class. To select child components, use > to replace the space between \#id and .class, for example, \#containerId>.content. |
The following is an example:
```
Title
Content
```
```
/* Page style xxx.css */
/* Set the style for all
components. */
div {
flex-direction: column;
}
/* Set the style for the component whose class is title. */
.title {
font-size: 30px;
}
/* Set the style for the component whose id is contentId. */
#contentId {
font-size: 20px;
}
/* Set padding for all components of the title or content class to 5 px. */
.title, .content {
padding: 5px;
}
/* Set the style for all texts of components whose class is container.
*/
.container text {
color: #007dff;
}
/* Set the style for direct descendant texts of components whose class is container.
*/
.container > text {
color: #fa2a2d;
}
```
The above style works as follows:
In the preceding example, .container text sets title and content to blue, and .container > text sets title to red. The two styles have the same priority, but .container > text declares the style later and overwrites the former style. (For details about the priority, see [Selector Specificity](#selector-specificity).)
## Selector Specificity
The specificity rule of the selectors complies with the W3C rule, which is only available for inline styles, id, class, tag, grandchild components, and child components. (Inline styles are those declared in the style attribute.)
When multiple selectors point to the same element, their priorities are as follows (in descending order): inline style > id > class > tag.
## Pseudo-classes
A CSS pseudo-class is a keyword added to a selector that specifies a special state of the selected element(s). For example, :disabled can be used to select the element whose disabled attribute is true.
In addition to a single pseudo-class, a combination of pseudo-classes is supported. For example, :focus:checked selects the element whose focus and checked attributes are both set to true. The following table lists the supported single pseudo-class in descending order of priority.
| Pseudo-class | Available Components | Description |
| -------- | -------- | -------- |
| :disabled | Components that support the disabled attribute | Selects the element whose disabled attribute is changed to true (unavailable for animation attributes). |
| :focus | Components that support the focusable attribute | Selects the element that takes focus (unavailable for animation attributes). |
| :active | Components that support the click event | Selects the element activated by a user. For example, a pressed button or a selected tab-bar (unavailable for animation attributes). |
| :waiting | button | Selects the element whose waiting attribute is true (unavailable for animation attributes). |
| :checked | input[type="checkbox", type="radio"], and switch | Selects the element whose checked attribute is true (unavailable for animation attributes). |
| :hover6+ | Components that support the mouseover event | Selects the element that the cursor is on. |
The following is an example for you to use the :active pseudo-class to control the style when a user presses the button.
```
```
```
/* index.css */
.button:active {
background-color: #888888;/* After the button is activated, the background color is changed to #888888. */
}
```
>  **NOTE**:
> Pseudo-classes are not supported for the component and its child components, including,