I think it's worth noting the HTML meta viewport tag:
<meta name="viewport" content="width=device-width, initial-scale=1">
Which allows developers to more easily program responsive websites and web apps on high res devices.
What @Rafael said:
A retina device declares one thing and does another
is exactly right... i.e. CSS pixels are not always equal to the actual rendered pixels. I would take a look at this article and the "A Pixel is Not a Pixel" section of the meta viewport article.
As @Rafael also mentioned, fonts are vector files so they will look good (for the most part) no matter how they are rendered.
As for the images, websites can be programmed to swap out an image for a higher quality version (2x or 3x) if a retina device is detected. This is why you would provide multiple to the developer. You can also, depending on what's in your image, save it as an SVG, Scalable Vector Graphic, which will make the developer's job a bit easier as he or she can just use one image file.
This might be too much detail for you, but I figured I'd add it for those who are interested in more details. Even as a designer I think it's good to know, at least at a high level, what's going on in the browser.