HTML - How to include external html page
<p>I am developing a website , I have an html page which has some contents, have also made a signUp modal in the same page, but the code is getting longer, I just want to know if I keep the code of sign up modal in another file with .html/.htm extension, and on button click that modal is displayed on the same page, not to be redirected to another page, like it is displaying on the home.
I don't want the home contents to be shattered , just the modal to be displayed on above. </p>