Introduction:

Universal Identifier (ur1) is a character class that is used in CSS to define a specific character sequence that can be used as a variable-length identifier. It is a legacy class that was previously used in CSS 3, but was later removed as it was not widely supported.

The purpose of usingur1 in CSS is to provide a more flexible and flexible way to define identifier names, as it allows for a variable length name without the need for complex rules. This can be useful when working with complex CSS rules or when using multiple CSS files.

1. Theur1 class can be used to define a variable-length identifier by providing a length value that is greater than 0. The length value is separated by any number of characters (except for spaces), followed by theur1 class名.

.my-class:first-child {

color: red;

上面的代碼中，my-class是ur1的類名，first-child是父元素的選擇器，兩個選擇器都使用了ur1 class。

2. Theur1 class名 can be used with any CSS property, including color, background, background-color, font-size, font-family, etc.

.my-class {

color: blue;

background-color: green;

上面的代碼中，my-class是ur1的類名，分別設置了顏色和背景顏色。

3. If the length of theur1 identifier is less than 0, it will be defined as an empty string (').

.my-class:last-child {

color: red;

上面的代碼中，my-class是ur1的類名，last-child是父元素的選擇器，因為父元素沒有子元素，所以使用了空格作為選擇器。

Conclusion:

Universal Identifier (ur1) is a character class that can be used in CSS to define a variable-length identifier. It provides a flexible and flexible way to define identifier names and can be useful when working with complex CSS rules or when using multiple CSS files. However, it should be noted that the support forur1 class is not very widespread, and it may not work in all browsers.