Better to start from scratch or Zen?

I really enjoyed building my first theme from scratch. but thats just me...I also had the html and css files built prior. what i did was just reference what needed to be referenced from a completely finished theme. (doesnt haveto be zen, could be any theme) Like the 'main menu' 'regions' a custom front page...etc.
-let us know what comes of it

After having done it various different ways including almost from scratch, adapting "Garland", "Framework" & "Clean" and recently started using the Zen theme my advice would be to use the Zen theme. Follow the instructions for creating a new Zen based theme from "STARTER KIT". When building from scratch I've found too easy to leave out important bits of code (tabs for example) which is not always clear why it is necessary when starting out. Zen is very well commented and really helps in understanding what's going on.

Initially we would try and clean up all the tags and nested divs that Drupal would put out in an attempt to have the same type of semantic markup we were used to writing, but that becomes a lot of hard unneccesary work; better to tweak what Drupal puts out naturally than re-write/theme it in many cases. Use your existing HTML/CSS to assist you in getting Zen to look the same applying the same rules to the Drupal/Zen class name which already exist. Firebug is most useful. Read through one of the new books written about Drupal theming (Drupal 6 Themes or Front End Drupal http://drupal.org/books ) lots of good info in them.
I'll probably be starting with Zen from now on.

I would take Zen, but to be honest, I think you should have asked this question before creating xhtml and css. IMHO, when you're developing with Drupal, it doesn't make sense to start with building xhtml/css/js. Myself, I usually build a fully functional site before I even enable the Zen theme. I explained why in this comment: http://drupal.org/node/456642#comment-1564728.

One thing you have to understand about Drupal theming, is that not all xhtml/css/js output is generated by the theme, because modules provide their own default output. The theme is able to override that output, but in many cases there is no reason to do that because the default output is usually valid, functional and flexible. If you want the final xhtml output to be 100% identical to the xhtml you created, you will have to override every single theme function and template... now that would be a waste of time. One way or the other, you will now have to merge your xhtml and css with Zen (or any other theme for that matter, but Zen is an excellent choice).

Drupal has ways of inserting functionality (ie. as tabs, messages, or stuff in the $closure variable) in the page that you might not be aware of. You are doing yourself a favor when you use a theme like Zen as starting point: In a Zen based theme, it's easy to cut out things you don't need, while in a theme-from-scratch, it's much harder to retrieve functionality that was omitted from the beginning.

I hope that helps.

I've gone every route possible in the past 2.x years haha :D

If you start from scratch (I wrote about the basic process), assuming you do include Every relevant PHP variable in your page.tpl.php (e.g. have a look at Zen or other starter themes, or this listing), then Drupal will still output the core styled version of everything. It may not look pretty, but it is all there. Once you see it on the page, you can use Firebug to find out what class or ID it has, then search in Zen or other starter themes to get some nice code to pull out for styling that (or if you're adept with CSS and prefer to do it custom, you can do it yourself). All the HTML is there on the rendered page, with classes/etc for you to look up. Most often if doing a theme from scratch I'll go the first route, finding what something is with Firebug, then getting CSS for that element from Zen or another theme. Sometimes the theme demands unique styling of things like the secondary tabs/menu, so I will do them from scratch.

When it comes down to it there's not "too" many elements you may prefer to copy like this - primarily things like secondary tabs (the ones you see when logged in), improved styling for the admin section and jQuery toggle fieldsets, etc. You should definitely be aware of how Drupal menus work (they are nested unordered lists, with classes dynamically added for active state, trail, etc). You might like to add the Menu Class module for more human-friendly classes on menu items. In any case you will have to be a little flexible with the final HTML and CSS (don't go overboard trying to force Drupal to use your precise original code... most of the time you "can", but really you should not). Drupal does have a touch of "div-itus" and adds extra classes a purist will probably not prefer, in order to make it easier for non-CSS-gurus to style, but if you can live with them, then life's easier :) If the theme is from a client and comes pre-coded, assuming the code is of good quality, I'd probably stick with adapting it rather than reworking it from a starter theme. If the code is lousy, then definitely toss it in favor of a good starter theme (it's much faster to adapt a starter theme to look like the old theme than it is to deal with a plethora of bad code and the issues it causes).

All that said... if you don't have good HTML and CSS ahead of time, then using a starter theme can be preferable in most cases. For my personal way of working, I like the Framework theme at the moment. I'm also looking into the Blueprint and 960 themes (which are for two of the main grid CSS frameworks). In every case though, there are a few gems from Zen that I adapt into any theme (e.g. excellent body classes in template.php). Especially if you're not an expert with cross-browser compatibility, most starter themes help you avoid the pitfalls of various browser issues. If you work as part of a team, it can also be great, since everyone gets used to the same theme framework and better understands how it works.

Hope this helps :)

Bah humbug to these guys saying it doesnt make sense to start with HTML/CSS etc, these guys obviously do not work as professional themers, since thats how the rest of the world works, designers and theme developers do this all the time and thats what they send me to be ported to Drupal.

I much prefer the Genesis starter theme, or if I'm working with a grids design I'll use Ninesixty.

Its actaully really easy to port your design over - if your a gun with HTML/CSS then fire up up your theme in Firefox, and fire up your Drupal installation with a subtheme ready to burn. You've already written the CSS so thats a big advantage (we are just theming elements & selectors here, its all pretty much the same bar the switch in selector).

This is where Genesis has it over Zen in a big way, because Genesis defines the vast majority of selectors and elements in pre-built modular stylesheets, so they show in Firebug, so element for element you can easily see where you need to apply your existing styles. Just use firebug to look at your own stuff, then figure out which selector to use in Genesis using the inspect element tool in firebug. Zen defines a few selectors but no where near as many.

Take it from someone whose been porting themes from professional designers and every other conceivable CMS/website/what-ever for the last 3.5 years solid that porting your design to Drupal with Genesis is a breeze.

Genesis should give you an easier upgrade path to D7 also because all the body, node, comment and block classes are identical to Drupal 7.

All this said, dude, it really depends on your design as well, if your doing something way outside the norm then you may be better off using your own layout and inserting in the Drupal variables in the right place. So, it just depends.

I am a drupal newbie myself, tried to build a theme of zen with no luck. I could not get zen to do what I want it to do, and ended up creating my theme from scratch. So I would recommend to do it from scratch.