Wikipedia:Page widening

Page widening of a web page is any case where the browser has to provide horizontal scrolling but by better page design, this scrolling could be avoided. For example, the line below is an intentional case: There is no need to do horizontal scroll - the rest of this line is standard filler text &mdash;&mdash;&mdash; At the HTML level, page widening can be caused by the use of the &lt;pre> tag which specifies preformatted text, ie. the browser honours the new line codes in the HTML and does no line wrapping of its own. In the MediaWiki context, unintentional &lt;pre> text is caused by the simple error (in the wikimarkup) of starting a new line with one or more spaces.

Advice
Always review your edits - preferably by using "show preview" but certainly after saving. You will not see page widening in the edit box - only in the rendered text. Page widening sticks out a mile - even if you do not see the scroll bar that has appeared at the bottom of the window, you will see a mono-spaced font with a dotted border.