Smashing lists 10 guidelines and tips for usability, covering everything from white space to scrolling: “Whether you’re working on a portfolio website, online store or Web app, making your pages easy and enjoyable for your visitors to use is key. Many studies have been done over the years on various aspects of Web and interface design, and the findings are valuable in helping us improve our work.”
Regarding white space and comprehension:
Most designers know the value of white space, which is the empty space between paragraphs, pictures, buttons and other items on the page. White space de-clutters a page by giving items room to breathe. We can also group items together by decreasing the space between them and increasing the space between them and other items on the page. This is important for showing relationships between items (e.g. showing that this button applies to this set of items) and building a hierarchy of elements on the page.