Choosing a proper typeface is one of the most important starting points when dealing with the design of a new website, and at the same time, it's still a field in which substantial mistakes can be made. Developers with little or no design knowledge may assume that only the standard MS Word fonts should be used, thus opting for Arial or Verdana. While professional designers will simply reject that choice since there are many other fonts which look much better for use in body copy.
Let’s make it clear: There’s nothing terribly wrong with Arial or Verdana since they were designed to look sharp and clear on a monitor even at small sizes. Often we cannot say the same for other print designers’ top-pick fonts that on a monitor look messy and blurred if used in small sizes. But Arial and Verdana bring two problems: the first is that they don’t look alike on Windows and MacOS, and second is that they are so widely used (often in unspecialized environments) that they will make your website look like millions of other websites from the past (and not that professional, right?).
So far, the best option is to use a free web font, even better if served by a CDN, like Google Fonts. Open Sans, Source Sans Pro, Ubuntu, Roboto and many others are really good fonts that look consistent on Windows and MacOS. They are easy to implement too; just keep in mind that, since they are so good, everybody is using them now, and in a short span (trends are running fast, you know) they will eventually look as boring as Arial and Verdana. So why not buy your font? There are many inexpensive and fantastic fonts available, designed specifically for web use, that for less than 50$ will give you that spark of individuality that will make you stand out.
One last hint: A lot of designers and developers are working on MacOS and don’t bear in mind that ClearType, the text-rendering engine of Windows, produces slightly thinner fonts. So, if you choose one of these super-stylish-ultra-thin fonts, use it only for big headlines; otherwise, it will be almost unreadable. And don't forget to run real cross-browser testing since it’s the only reliable method if you want to target the widest audience in the best way possible.