One example is in control over line length. A line of about nine or ten words is usually considered to be optimal for comfortable reading. On the web, there is no way to ensure that. Many designers try, by formatting text within a table that forces a fixed width. However, the table width must be specified in pixels, which has no relationship to the number of words, or even to the size of the type used to present those words.
Typographical controls are available, but are unreliable to the point of being counter-productive.
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.