下の文章の空白は、すべて通常の空白を使っています。

Sometimes authors may want to prevent a line break from occurring between two words. The   entity (  or  ) acts as a space where user agents should not cause a line break.

下の文章の空白は、赤色で示す「between two words.」の空白に「 」を使っています。

Sometimes authors may want to prevent a line break from occurring between two words. The   entity (  or  ) acts as a space where user agents should not cause a line break.