CSS Selectors: CSS Selectors are used to selecting HTML elements based on their element name, id, attributes, etc. It can select one or more elements simultaneously.

element selector: The element selector in CSS is used to select HTML elements which are required to be styled. In a selector declaration, there is the name of the HTML element, and the CSS properties which are to be applied to that element is written inside the brackets {}.

Syntax:

element_name {
// CSS Property
}

id selector: The #id selector is used to set the style of the given id. The id attribute is the unique identifier in an HTML document. The id selector is used with a # character.

Syntax:

#id_name {
// CSS Property
}

class selector: The .class selector is used to select all elements which belong to a particular class attribute. To select the elements with a particular class, use the (.) character with specifying the class name. The class name is mostly used to set the CSS property to the given class.

Syntax:

.class_name {
// CSS Property
}

Cascading Style Sheet(CSS) is used to set the style in web pages that contain HTML elements. It sets the background color, font size, font family, color, … etc properties of elements on a web page.
There are three types of CSS which are given below:

Inline CSS: Inline CSS contains the CSS property in the body section attached with the element known as inline CSS. This kind of style is specified within an HTML tag using the style attribute.
Internal or Embedded CSS: This can be used when a single HTML document must be styled uniquely. The CSS ruleset should be within the HTML file in the head section i.e the CSS is embedded within the HTML file.
External CSS: External CSS contains a separate CSS file which contains only style property with the help of tag attributes (For example class, id, heading, … etc). CSS property is written in a separate file with .css extension and should be linked to the HTML document using the link tag. This means that for each element, style can be set only once and that will be applied across web pages.

CSS plays an important role, by using CSS you simply got to specify a repeated style for an element once & use it multiple times because CSS will automatically apply the required styles.
The main advantage of CSS is that style is applied consistently across a variety of sites. One instruction can control several areas which are advantageous.
Web designers need to use a few lines of programming for every page improving site speed.
Cascading sheet not only simplifies website development but also simplifies maintenance as a change of one line of code affects the whole website and maintenance time.
It is less complex therefore the effort is significantly reduced.
It helps to form spontaneous and consistent changes.
CSS changes are device friendly. With people employing a batch of various range of smart devices to access websites over the web, there’s a requirement for responsive web design.
It has the power for re-positioning. It helps us to determine the changes within the position of web elements that are there on the page.
These bandwidth savings are substantial figures of insignificant tags that are indistinct from a mess of pages.
Easy for the user to customize the online page
It reduces the file transfer size.