Create A Sticky Footer With CSS
A common problem with this layout is that some pages are shorter than others, which means the footer doesn't stay at the bottom of the page. Instead, it rides up ride below the content, and on short pages, it's not a good look.
You might be tempted to set the footer to an absolute or fixed position, but we don't want the footer to be visible at all times if the page is long enough to scroll. We want the footer to get pushed down below the content on those pages.
You can check browser support for Flexbox on Can I Use.