The images that you see here represent posters achieved entirely using lines of HTML and CSS code. You can craft such visuals directly in the browser with no need for a digital tool such as Photoshop, unless you want to create a mock version first.

These are some not so complicated or complex examples but the idea is that you can do such thing with code. This is not a replacement for what you can achieve with the regular digital tools but it’s a new and interesting way to deal with visuals on the web. This is definitely a way to push things forward, experiment and play with CSS3 capabilities.

Disadvantages? You can’t compete with an image achieved in Photoshop and if coding is not your strength then it may not be an option.

Advantages? Well, you work directly in the browser and I’m sure the browser loads faster. Also you can achieve specific effects inside the image such as: changing color when hovering or some animation that otherwise you can’t produce. (unless you use Flash).
If you’re just starting out with CSS, then I highly recommend this type of play to learn the beauty of CSS and come up with something interesting and original. See below some examples, tutorials and advice form the web design community.

More Resources

Design Informer
Nicolas Gallagher
Boag World