I am newbie here and I need some help related to HTML coding,

Few months ago I created my first site and when put my site for site-checkup tool I got an error which says (Your site using useless HTML coding which slow down your site speed) even though I tested my site on several browser's, But got the same result which actually slow down my site speed.

So What should I do to remove or something change in my site to solve this HTML error?

Thanks.