We expect a web developer to have a solid or even expert grasp of HTML and CSS-based layout practices. Generally, there is also a highly proficient level of knowledge and experience in one or more supporting technologies: JavaScript, PHP, Ruby, etc. Web developers are familiar with the technical aspects of web servers and the types of applications and sites that run on them.
A web developer may or may not have a firm grip on design. The necessity of converting Photoshop designs from others means they almost always know their way around the apps used for web design but aren’t necessarily the people to ask to define the look and feel of your site.