What is the identifier of the id selector in css

In CSS, the identifier of the id selector is "#". You can specify a specific style for the HTML element marked with a specific id attribute value. The syntax structure is "#ID value {attribute: attribute value;}" . The ID attribute is unique and non-repeatable in the entire page; the ID attribute value should not start with a number. IDs starting with numbers will not work in Mozilla/Firefox browsers.

The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.

The id selector can specify a specific style for HTML elements marked with a specific id.

The HTML element uses the id attribute to set the id selector, and the id selector in CSS is defined with "#".

The syntax structure is as follows:

#ID {
  属性 : 属性值;
}

The following two id selectors, the first one can define the color of the element as red, and the second one can define the color of the element as Green:

In the following HTML code, the p element with the id attribute red is displayed in red, and the p element with the id attribute green is displayed in green:

It is worth noting:

• The ID attribute is the only one that is not repeatable in the entire page.

  An ID value can only appear once in an HTML document, that is, an ID can only uniquely identify one element (not a type of element, but an element).

  That is, the id of the element is unique in the page, so the id selector is used to select a unique element!

• Do not start the ID attribute value with a number. IDs starting with numbers will not work in Mozilla/Firefox browsers
  

id selector and derived selector

In modern layouts, id selectors are often used to create derived selectors.

The above style will only be applied to paragraphs that appear within the element with the id of sidebar. This element is most likely a div or table cell, although it may also be a Table or other block-level element. It can even be an inline element, such as or , but this usage is illegal because it cannot be embedded within the inline element

.

Multiple uses of one selector

Even if the element marked as sidebar can only appear once in the document, this id selector can also be used as a derived selector. Used many times:

Here, what is obviously different from other p elements on the page is that the p element inside the sidebar has been specially treated. At the same time, unlike other p elements on the page The obvious difference between all h2 elements is that the h2 elements in the sidebar also receive different special treatment.

Individual selectors

The id selector can function independently even if it is not used to create a derived selector:

According to this rule, the element with the id of sidebar will have a pixel-wide black dotted border, and there will be 10 pixels of wide padding (padding, content blank) around it. Older versions of Windows/IE browsers may ignore this rule unless you specifically define the element to which this selector belongs:

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