I don't know where you get your information from, but maybe your idea of Mac not supporting HTML is the fact that Microsoft didn't have a Mac version of Front Page. If you speak to any REAL web desiger, they will tell you that the truth is actually the other way around. As the web developed designers and programers worked to make web pages look more like printed material than a grid (which is basically how they are built). One of the first tools that gave a designer control over typography was CSS (Cascading Style Sheets) For the first time you could actually specify the size of your type rather than just bigger or smaller. The biggest problem was that Explorer YES MICROSOFT EXPLORER, and only the PC version did not support CSS and Microsoft had no plans to do anything about it. Ask any designer out there that has worked on websites since the 90's and they will tell you, Macs were not the problems. The problem was when you went to view it on a PC, especially in Explorer, you than got to see just how much the PC screwed up the page. There have been some workarounds like a small bit of code that will force Explorer to see your CSS. And the more advanced the programming gets, the more consistent a site will look on different browsers and on different operating systems. But there are some things we as designers will never be able to control and that is the user. If they have their computer set up for minimum resolution and 256 colors (yes, people still do that), than no matter what, the site will still not look as good as on a computer with the proper settings.
So don't go blaming the OS. The problems have more to do with poor designers, lack of knowlege of HTML, and browsers such as Explorer that think that because they are do big they don't have to bother keeping up with the changing technology.
One other thing to keep in mind, those plug and play programs such as Dreamweaver are great, and can be huge time savers, but without a knowledge of the actual programming language and scripts, you will always run into trouble. One missing bracket or quote mark can send a whole page out of whack.
And not to rub it in, but just because these programs have so many features and allow you some flashy things on your site, it doesn't mean you should. It goes back to the beginning days of desktop when all of a sudden small companies had their secretaries desiging their flyers. They were so busy with all the fancy effects, the point of the piece never got across. As the saying goes "Just because you can, doesn't mean you should."