The 'em' is a very useful unit in css, since it can adapt automatically to the font that the reader uses an em is relative to the current element it is defined on. If you use relative sizes (like 0. 9em), they multiply and can lead to unexpected dimensions. Now, the default size of a font is not standard between browsers. They have the same effect on normal web browser rendering engines, but there is a fundamental difference between them. As the author writes in a discussion list post: Think of three different situations: I heard you should define sizes and distances in your stylesheet with em instead of in pixels. So the question is why should i use em instead of px when defining styles in css? Is there a good exam. 即为此表情，代码为 [em]e3006 [/em] 1、当发送或接收出现问题时，表情可能显示为代码。 2、此表情适用范围仅限于 qq空间 中的留言板，聊天窗口无法显示。 3、此表情显示的画面是一个人物对着电脑屏幕流汗，同时手臂上下 机械运动。 4、图片已上传，测试qq版本为qq7. 5（15456）

Both of them emphasize text. Tag shows text as italics, whereas <strong> Is this the only difference? Em has the purpose of giving em phasis to the content. However, emphasis is semantic while italics is presentation. I was deprecated in favour of em to make html a truly semantic markup.

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