In the early days of the internet, there were 2 major browsers: Internet Explorer and Netscape. A browser is simply an application software that allows you to view information on the web. In those days, different browsers displayed pages differently and this posed problems for web site developers when creating web sites.
Today we have 4 major browsers: Internet Explorer, Chrome, Firefox and Safari.
In 1994 web standards started to evolve. Gradually over the years with improvements to html code along with the introduction of CSS (cascading style sheets for formatting your page) and changes to browser software, websites are now uniform in most cases.
I check your pages in all the major browsers in both Windows and Mac, to make sure they will be viewed correctly and look the same in every browser.
I use HTML 5 and CSS5, which are the latest standards for web design development.