DIV CSS learning--about ID and CLASS_html/css_WEB-ITnose

DIV CSS Learning--About ID and CLASS The method I recommend is to use underlines between words, or the method of splicing words. Because CSS is case sensitive. As you can see, all the download files I provide you are processed in lowercase.

We first deal with the style of the sidebar. After analysis, we can know that the sidebar has four parts. We divide these four parts into four layers:

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Here we have ID and CLASS, so there are My friend is about to ask, when should ID be used and when should CLASS be used?

　ID represents uniqueness and will only appear once in this page. We use it to represent the structure of the layout;
CLASS represents a group (class) or a class with the same properties. Elements, they can share styles and will appear multiple times on the page.

Since the styles of member login, site information, page standards and content words in these areas will be the same, we put them in a group.

But sometimes, when we need to set different styles for the elements in these layers, how do we do it? For example, the words "member login" are bolded with 14px, and the others are bolded with 12px. How do we write CSS? See the code below?
.bar_title {font-size:12px;font-weight:bold}
#login .bar_title {font-size:14px}

　#login .bar_title represents the login ID The CLASS under the layer is the style of the bar_title element, which means that the style can be customized for certain elements in a targeted manner. #login .bar_title When the style of font-size:14px is set separately, it also inherits the style of the .bar_title group (class), which is font-weight:bold, bold.

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