It can be all to easy to assume that style guides are only for logo’s and identities, but as Anna Debenham points out over on 24Ways.org—this is an excellent website that runs for the first 24 days of December with web based guest articles on all manor of web themed topics—that many websites may require, or would benefit from, a detailed front-end style guide.
Detailed and complex identity projects usually benefit from an identity style guide; this is to ensure that anyone who might be hands-on with the identity can ensure that their creative input is consistent, and within the scope of the original brand design.
So given the often detailed and complex CSS style structures of many websites it then makes sense that a front-end style guide is also created. This would be to ensure that any further application of the website design is within a pre-defined style.
No ones likes to see anything they have helped create suffer at the hands of people who don’t have these guidelines to follow. If you care about how your work is handled when you are not there to supervise then creating a one of these front-end style guides will be a huge blessing.
If you enjoy looking through brand identity guidelines then you will likely find this post useful: Bulging Sack of Brand Identity Guideline Resources