There is a standard for HTML code, but not really for web design.
There are different versions for the HTML standard. This creats problems as some browsers do not actually meet the standard. Some browsers even include extra things that are not in the stardard.
How do you evaluate a web designer? Well, if they have made a web site, there are a few things you can do.
Check if their web pages are valid web pages according to the standard. You can do that at this page:http://www.w3schools.com/site/site_valid…
If any errors come up, note how many errors. The person with the lowest amount of errors would be better. The person with no errors would be best.
If they have more than one page, it is better, as you can check for errors on each and every page they have made.
The next way to evaluate them, as long as they have made web pages, is to look at the web page(s) and see how easy it is to read. How easy it is to find things like a menu or navigation, etc.
Also check on the colours they use, as some pages are completely the wrong colours to use. For instance, the text may be orange, and the background colour is red. You would not easily see the text.
Also check whether the layout of the pages are any good. Some pages you have to go down the page to see the actual content because it is too wide to fit between two columns.
Then you could check if they are using CSS. If they are, are they using external, internal, or inline CSS. Going from the best to worst is external, internal then inline.
You could also check the source to see if they used a program like dreamweaver to make the web pages. These people may be good at point and cick development, but probably don’t know how to do it by hand.
Sometimes you need to know how to do it by hand in case you need to fix something,. or change it, etc. If they can’t do this, they need to learn more.
If this still does not help you, you could write some requirements for a web page and get the people to create the web page. Like a test.
See how long it takes them to make it. How good it is, with all the above things. Also check if it is valid through the validation page above.
After all of this, you should have a good idea of how the person is.

>> Is there a rating system for web developers?
No.
>> How do I effectively evaluate a web designer?
Very difficult.
Ask for a portfolio. What have they done already? But don’t assume that they had all that much to do with the projects they show you.
Ask for references, then CALL THEM.
There is a difference between a web developer and a web designer. You used both in your question. Make sure you know the differences and interview according to what your REAL needs are.