People use the terms web developer and web designer interchangeably when they shouldn't because they are completely different professions. The confusion comes from
poorly organized projects web developers doing a little web design and web designers doing a little development. In reality, the two work together to make a website. Their roles are just different.
The developers are responsible for writing the code and making sure that the website will function right. They get the design from the web designer and make it real. Web developers don't have to worry about usability
if the designers have done their job right because it's been built into the design.
The designers focus on the user flow through a website. They take into consideration where to place buttons, pictures, links, text, videos, and everything else you see on a web page. Web designers make the layout for everything on the website and they make it easy to use.
A web designer will focus on things like color, typography, and user experience. They use software like Photoshop or Sketch to layout their designs so they can hand them off to developers. Unlike a web developer, a web designer doesn't need to have any coding experience. It's helpful, but not necessary.
Of course no roles are never kept this clean. What really happens is that the web designer might need to know a little HTML and CSS to understand what the developer is doing. A web developer might need to know a little about web layouts because last minute changes aren't unusual.
Most good designers know at least a little HTML and CSS and most developers know a few of the best design practices. There are some unicorns out there that can do both really well, but for the most part it's best to focus on one or the other because they require different kinds of thinking.
Web development is a lot more logical in it's approach. There are rules to follow and an order in which things should be done
(whether they are done or not). Web design is a lot more artistic because you are trying to appeal to people's emotions and make your website stand out while still matching your brand.
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Thanks for reading! :)