class attribute can be defined within <style> tag or use in separate file .
See in Example:
This is first heading use of class elements
See in Example 2:
The class Attribute
Use CSS to style elements with the class name "city":
India is the capital of New Delhi.
India is the Grate Country.
See in Example :
Click the button to hide all elements with class name "city":
China has the largest population in the world.
India has the second largest population in the world.
United States has the third largest population in the world.
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