The Chosen Ones

i.e. those support as documented in the CL/XSL codebook:

Preferred:

1. Art Quality single page designs ¹: CSS Zen Garden, c.f in particular The Chosen One which is a skin article you can edit if logged in to set a specific official Garden style.
2. Drupal 6 Zen Themes²
3. Wikimedia skins (like this one EG).

Also supported:

1. Orbeon XForms with DCMS.
2. Template World templates.
3. WordPress themes.

A popular method of web development is to develop "mock-ups" with a given graphical treatment to be supported by backend software. CL-XSL supports this for mockups using the above CSS templating and DCMS supported packages.
In a development process which is all of literate, considered, and (having gotten) real, a mock is essential a pre-production version of the contracted thing.

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¹ In general, unless released under some license, these always require cooperation/permission of original author.
² My generic applications use Zen wireframes and an authenticated use can select any installed Zen theme.