Nowdays, life is simpler thanks to a few tools that convert graphic designs into CSS and HTML code.
First, there are designing tools that produce code directly - a kind of Dreamweavers of the responsive age. They include WebFlow (webflow.com) and Macaw (macaw.co), to give you two examples.
A slightly different approach can be seen when using tools that add export functionality to traditional graphic editors. A typical example of this is CSS Hat (csshat.com). Today’s predominant design tools such as Adobe Photoshop and Bohemian Coding Sketch are doing export to CSS by themselves (bohemiancoding.com/sketch).
Finally, there are tools which connect the worlds of graphic designers and coders. Let’s mention Avocode (avocode.com) and Brackets by Adobe (brackets.io).