块级元素 vs 行内元素
Block element vs Inline element
一句话：块级元素独占一行、宽高可控；行内元素跟着文字流走、宽高由内容决定。
具体差别就三处，记住这三条就够答：
- 换行—— 块级前后自动断行（
div、p、h1、ul）；行内不断行（span、a、strong、img）。 - 宽高—— 块级可以设
width/height；行内设了不生效。 - 内外边距—— 块级四个方向都生效；行内的上下 margin 不生效，上下 padding 视觉上会溢出但不撑开行高。
会追问：「那 inline-block 呢？」 —— 它是折中：不换行（像行内），但宽高和上下 margin 都生效（像块级）。导航按钮常用它。
还会追问：「img 是行内元素，为什么能设宽高？」—— 因为它是替换元素（replaced element），内容由外部资源决定，浏览器对它网开一面。input、video 同理。这个点答出来会加分。
In one line: a block element takes the whole line and you can set its width and height; an inline element flows with the text and is sized by its content.
There are exactly three differences worth remembering:
- Line breaks — block elements break before and after (
div,p,h1,ul); inline ones do not (span,a,strong,img). - Width and height — settable on block elements, ignored on inline ones.
- Margin and padding — all four sides work on block elements; on inline elements vertical margin does nothing, and vertical padding overflows visually without pushing the line apart.
Follow-up: “What about inline-block?” — it is the compromise: no line break (like inline) but width, height and vertical margin all work (like block). Nav buttons use it constantly.
Also asked: “img is inline, so why can you set its size?” — because it is a replaced element: its content comes from an external resource, so the browser makes an exception. Same for input and video. Getting this one right scores points.