I agree with Nina its more about knowning DNN then it is how to skin. I do slightly disagree with making compliant skins is difficult regardless of the difficulty. I can typically take a very complex website design that uses tables and create the same thing with a fraction of the code that is compliant. The problem is as nina states each browser has a slightly different interpetation of CSS, however IE7 / IE 8 have resolved most of the nightmare issues granted we will have a few more years to go with people using IE6. Overall though you just create an IE hack.css file and use a small conditional comment to pull in the IEHacks.Css tid bits when needed. Not that much needs to be Hacked anyways. Mainly just a few spacing issues.
I have to meet strict compliancy standards at work, and its really easy to do. Just takes strong knowledge of how to code html layouts that use CSS for positions instead of using tables for layout. Once you get the hang of it you can out build any table based layout with far less code and much greater control over it. Just have to realize pure CSS based layouts require you to think with your mind and less with your eyes.
Now I really suggest going through all the DNNcreative skin tutorials if you really want to get into pure CSS layouts / desing or just skinning in general and want to have a guide holding your hand during the process. Lee does a great job with skinning, and really focuses on being compliant from start to finish. The think I like about his tuturials is he doesn't spend a ton of time on the layout of the site, just a general overview, from there he goes into lots of detail around how to use your skin's CSS to control headers, footers, html text, menu text etc. Which is where most people start to have troubles with their skins. Sure they managed to make a skin with 4 panes that has a horizontal menu, and breadcrumbs, but the containters headline text is black when it should really be white, and the pages background is black, but all the html text is defaulting to black. After watching ALL the tuturials you will have the ground bases to really start to understand, that DNN skinning is more about knowing DNN then how to skin. I say all cause his basic skinning tutorials cover design concepts and lots of little quirks you see in DNN. IE how to deal with a background image when you have a menu / module that is going to dynamically expand past your set image size.
Tokens / Attributes are a must, and using html / xml / css vs aspx / css is really just preference its not like xml is that hard to pick up for DNN all the tokens / attributes follow one basic structure from there its just reading the documentation to know what values to set for what token. So don't let the xml scare you. On the flip side building ascx skins is just as easy as html / xml / css skins. It's completely about preference.
Now for CSS menus I agree with Nina if you need a really complex Navigation system that has very specific needs you can drop the cash on Snapis's menu. I however use House of Nuke's HouseMenu its free and uses unordered lists + CSS to display. It can do most to everything Snapis's can you just have to be creative at times. It also does it without the larger overhead using snapis's can create. Once again its all about preference / needs and knowning DNN's structure. Also note that the next release of DNN should at some point have a new menu version which can render CSS friendly menus, so we might get a built in skinobject to manage pure CSS menus. The use of menu modules in DNN can work very nicely I use mandeep's CSS Tabs module all the time for a "menu" system. I works nicely cause of the controls you can use to limit access, but it's still with draw backs of administration of the tabs. IE not best option for a navigation menu for the main site.
In regards to DNNCreative's site all the tuturials there still have valid merrit to being used, very few things if any are out-dated, and from all the books / tutorials I have seen / used I have to say his is worth every penny, cause he really covers the what is actually going on aspects vs "here is how you do it" copy me and that's it.