So, I created my whole site and all the pages for desktop, then make the tablet, and then the cell phone.
But I went to watch some videos about making a responsive site, and the guy said that he uses a term " mobile first ", and if he starts to create the site in the size of the cell phone and then increase .
Is this mandatory? How do I make it responsive if I already have the site 100% ready for desktop?
And one more question : I realized that responsiveness is only in CSS. Is it possible to change anything in HTML to be viewed only when you're on the mobile or tablet screen?