Other examples include forcing paragraph indentation with invisible images, or using images or markup to emphasize capital letters at the start of paragraphs. In general, the presentation environment is not predictable. Window size for the browser will vary. Different systems handle color and type differently, with significant differences in contrast and legibility. Browsers handle presentation in different ways, and offer readers varying capabilities to override an author's choices.
Resolution is much less than for printed documents.
And perhaps most subtle of all, presentation occurs within an application, so that presentation becomes interactive. The solution to these problems is to learn what can be controlled, and what cannot. In general, presentation is the browser's responsibility, but in turn it can only base presentation on the HTML markup provided by the author. The new capabilities available through the web also require some relearning. Typically, when a reader arrives at a page, the designer already has the reader's attention.