If you want to understand why you should even bother to use YAML, this page is for you. We discovered that many web designers didn't see the evil of table based layouts. For those web designers we assembled the 10+ Reasons for CSS-Layouts. For those web designers that have seen the light but chose to use a DIY approach instead of using the power of a framework, we assembled the 5 Reasons for a CSS-Framework. And lastly, the choice of framework is nothing but trivial so we summarized our research at the bottom.

1. SEO (Search Engine Optimization) work better with CSS Layouts
2. Design flexibility - you are more flexible to introduce changes in a layer based design than with tables
3. Clean code and structure - your HTML is much easier to edit, because it's so lean and clean. Plus, table-layouts are usually a mess of tables-in-tables-in-tables. Another bonus: better separation of concerns, when working in teams.
4. Cross-browser - once you understand how to user layers, it becomes easy to establish the same look accros browsers
5. iPhone - ever wanted to optimize for a mobile device and found it hard or impossible with tables? Guess what, it is impossible! With a clean CSS-layout it's much easier!
6. Animation and effects - creating visually appealing websites is easier when using layers, say with jQuery
7. Accessibility - yes, most of our customers are not disabled - or at least noch much so. In many countries and industries (like government related sites in Switzerland) you are required to create websites according to accessibility standards. Also, think about "how does my website work without JavaScript"?
8. Usability - you may think that things that look alike, are alike. Think again. For example, printer-friendly websites. We also recommend useit for reading. 
9. Web Standards matter - Some very smart people thought all the issues through and determined that table layouts are a problem. If you disagree, I would recommend researching it some more. 
10. Future proof - if you're old enough to have experienced the Dot Com Bust, you also know what browser generations mean - rewriting every page, again and again. Working with the official standards will save you that unproductive work.
11. Professionality - you might be able to get away with this, since your customers often don't know the difference. But won't your customers hate you, when they find out a year later that you made their layout using something that violates the standards? How will you justify yourself when your customer gets sued for not adhering to accessibility regulations or loses customers because Google doesn't rank his site well?

More similar stuff: CSS vs. Tables, 13 Reasons or Top 10 Reasons why CSS is better than HTML tables

1. Higher productivity - you are more productive because the framework does much of the work for you - for example clean separation of concerns, printability, etc.
2. Less mistakes and easier debugging - as the framework solves many issues like browser specific bugs it saves you much time (=money)
3. Knowledge in the box - yes, you should understand what you do, but in 90% of the cases you'll be glad it works without you having to know exactly why this bug didn't appear
4. Adhering to best practices - don't re-invent the wheel, use tested and proven methods
5. Community - when you're using the same tools as a large group of motivated individuals, you will find it easier to find exact instructions to solve a specific issue and it's easier to ask for help

I hope all the above convinced you of

1. The requirement to use CSS based layouts
2. The benefits of using a CSS-Framework
3. The advantages of prefering YAML compared to Blueprint or YUI

So why should everybody using DotNetNuke use the same framework? First of all, this is not a rule, it's a recommendation. You can use any framework you choose, just like you can opt to use Mootools instead of jQuery. Here are the top reasons why all benefit when going for a single CSS-Framework:

• When you think about the Community, you'll see that people can help each other much better when we all use the same standard.
• Convention over Configuration - saves you time, and saves you many problems
• Everybody wants to make every mistake only once, and ideally the person doing the mistake is someone else, and you can learn from his experience. This is only possible if we all use the same tools
• Compatibility - every framework needs some adaptions and testing to get all the small incompatibilities worked out. 2sic Internet Solutions will maintain this YAML Template for DotNetNuke, so that you'll always have a current working version.
• Interoperability - getting a framework like DotNetNuke to work with a framework like YAML takes some work. Making sure it works with a standard JavaScript framework takes more work, and the more frameworks and standards appear (maybe the next one will be a Flash framework), the bigger the need for interoperability out-of-the-box. When everybody uses the same set of frameworks, this will be the case. A bit like Apple, where everything comes from one source :)
• Collaboration - imagine different people in a team or even different agencies working together, using the same convention - what a wonderfull world! The iPhone guy, the Flash specialist, the jQuery man and the SEO pro all understand what the others did!