If you look at all the pages of a particular site, you will notice that most pages share common parts. These common parts typically include navigation, header, footer, styles and so on. Of course, you can duplicate the common part in every html file on that site. But as the site grows, making a change to one of the common parts ends up in a lot of tedious work: you need to apply the change to every single HTML file on the site. Now imagine your site has 1000 pages! You don't want to do that.
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