I'm the lead web developer for a small design company with some big clients.
Not only are guidelines like this common, but this isn't half as detailed as some of the stuff we get handed.
Working in a company of designers is kind of funny sometimes. They'll think of technological problems in those fuzzy, vague terms of non-techies, but once the conversation becomes about "branding", out comes a razor-sharp eye for detail.
Here's the Skype Brand Book, which a complex version of the linked style guide. All big companies have something similar to this, but Skype's is particularly well done.
I'm still waiting to see the new branding book for the company I work at. I thought the old branding sucked, but I was very pleased with the quality of the branding book, and I'm "just" a dev (a back-end one at that).
I have worked for a university and several organizations that have had basic guidelines like this. It's much more important in large organizations when many small departments or individuals are creating branded things.
One sees this basic type of thing in press kits and even (er, kinda) "link to us" or "powered by" graphical widgets, but the focus on consistent color and typography are things I think could stand to get some more attention. This type of reference is especially helpful for non-designers who aren't immediately inclined to think about consistency and branding.
I have worked for a university as well, and multiple logos I designed for the small grant that I worked for were shot down for misusing the official logo somehow. The review process seemed very politicized, subjective, and opaque. It would have been nice to have detailed guidelines (See dcurtis' link) that spelled everything out. I came away with, "your logo is round and so is our official logo - this can't work, sorry."
Most major companies have these sort of guidelines.
When I used to work at Ericsson, there was a huge manual with all the design guidelines that specified everything from the exact shade of blue for the Ericsson logo, the font, and to how far the 'three sausages' logo should be spaced from the word 'Ericsson' (for the record, turn the 'E' horizontally and place it between the 'n' and the logo, that's the correct spacing).
I love reading design guides, but even more so the style guides that various newspapers use. For example, here's the Guardian's style guide for words that begin with the letter H:
Every company should have this. My day-to-day job is helping companies role out their brand guidelines and we have seen the benefits of having guidelines available. Putting aside the value of having a consistent brand, we have seen big increases in productivity and employee satisfaction just because a). they aren't starting from scratch each time and b). there are templates or assets available to help them out. Add the ability for employees to share their work and experience with each other and you can really get some work done.
Even if you are using vendors to help out with your design, it's a great way to lower their time spent (and your bill) by more rapidly getting them up to speed.
I'll echo everyone else here that has said a lot of websites and companies do have this.
But the reason not every one of them does is that it is extremely time consuming, takes a lot of thought, and you have to spend time enforcing the guidelines once they're created. If you're not committed to following them, it's a waste of time and money to define them.
Most start ups are worried about things other than logos and colors matching across multiple mediums (print, web, broadcast, etc.) because most web sites and web start ups only have one medium: the browser.
People who always ask why you should pay hundreds or thousands for a corporate identity should take a look of the amount of work behind those guidelines...
seriously good advice. And if you are not a huge company, just making a single page with a list of colors and logos is a huge help and doesn't take more than an hour to put together.
I am barely more than a hobbyist and I have a little one-page guide for my own reference, mostly to sample colors easily.
Using a consistent color and style create a consistent look and feel for a brand. That consistently becomes something people recognize, and that's a good thing.
Not only are guidelines like this common, but this isn't half as detailed as some of the stuff we get handed.
Working in a company of designers is kind of funny sometimes. They'll think of technological problems in those fuzzy, vague terms of non-techies, but once the conversation becomes about "branding", out comes a razor-sharp eye for detail.