The Web is not paper.
No matter how many times that statement is repeated (and many now consider it a cliché), there are still many so-called designers who think they can design web sites the way they design printed pages. Here are some examples of what happens when web coders do not take into account the fact that a web browser can have different settings from theirs.
The problems are usually caused by over specification: specifying a size when none is necessary. As a result, when the the viewer uses a larger font size, text overflows its container and often overwrites other text.
Justified text is usually inappropriate on a web page, and it is
made worse when the columns are narrow (usually due to width
specification in px instead of em or %).