The problem is that web development is not that simple – no browser is 100% standard compliant – for any browser you need to do workarounds. If you only test your site with Internet Explorer, then you will get an Internet Explorer only web site. If you only test your side with Mozilla Firefox, then you will get a Firefox only web site (I know, because I’ve done this a lot).

Making a site that works with all browsers is not simply writing according to some standard – its about testing your web site, the entire web site which can be thousands of page, with all the relevant web browsers. Which is a lot of work.