September 10, 2011 update: This checklist is now outdated. The most updated checklist in the Codex is available here.
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1. Development & Coding

Header

Footer

Error Handling

Syntax & File Handling

→ svn

→ JavaScript

2. Formatting, Function, and Logic

Navigation

Widgets

Pages

Posts

Multiple Page Posts

The <!--more --> tag

Enclosures

Video

Images

Gravatars

Categories

Tags

Comments

→ No comments

→ Disabled Comments

→ Trackbacks

→ Comment Form

3. Theme Unit Tests

Download and import the test data here: WordPress.org Dummy Content. The following tests are to be done on Posts with similar titles.

Posts

→ Future Post

→ Layout Test

→ Simple Gallery Test

→ Category Name Clash

→ Test with Enclosures

→ Block quotes

→ Many Categories

→ Many Tags

→ Tags A and C / Tags B and C / Tags A and B / Tag C / Tag B / Tag A / Tags A, B, C

→ Raw HTML code

→ Simple markup test

→ Embedded Video

→ Contributor post, approved

→ Embedded Video

→ One comment

→ No comments

→ Many Trackbacks

→ One Trackback

→ Comment test

→ A post with multiple pages

→ An article with a More tag

→ Cat C / Cat B / Cat A / Cats A and C / Cats B and C / Cats A and B / Cats A, B, C

→ This post has no body

→ http://yoursite.com/2007/09/04/14/ ("This post has no title")

→ Protected: Test with secret password and excerpt / Protected: Test with secret password

→ Image align test

→ Test with thumbnails

→ Test with wide image resized

→ Test with wide image

→ Test with image

→ Post with categories

→ GMT+10 test post / GMT test post / GMT-6 test post / Article in the distant past

→ Draft Post

→ Draft post with file attached

→ Contributor post, pending approval

Pages

→ About / Lorem Ipsum

→ Page with comments

→ Page with comments disabled

→ Parent page / Child page 1 / Child page 2

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