First, almost a religious requirement for some. Personally, compared it to a white shirt. So, white background for clean web design. Given, proper Netiquette to be neat.
White Background Netiquette For Web Design
After all, makes it easier to read colored text. Also, display graphics on screen. And so, a common background for all digital document types.
To start, text is usually colored. Black is the most popular text color. Then, shows up well. As a result, their is a big contrast in colors.
Graphics generally have many colors. The many colors of graphics display nicely contrasted with a white background. In general, digital documents have white backgrounds, word processing text files, graphic files, and others. A web page is a digital document.
Good Web Design
White backgrounds for web content are considered clean. This is true for graphics and web pages. The top websites in the world ranked by Alexa have white backgrounds.
This means, if your site is clean. Given, following the standard set by the most popular websites.
White Background Netiquette
Use white backgrounds because content shows up clearly.