If a sibling block box (that does not float and is not absolutely positioned) follows the run-in box, the run-in box becomes the first inline box of the block box.
如果兄弟元素不是浮动和绝对定位，那么就会表现得像紧接着那个块元素的第一个行内元素
A run-in cannot run in to a block that already starts with a run-in or that itself is a run-in.
不可以跑进已经由run in开头，或者本省就是run in的元素里
If the run-in element contains anything block-level, it becomes block-level
可以让标题和后面的文字出现在同一行，不用牺牲语义化，用其他方式实现容易遇到各种问题
h3{display:run-in}
例如用float来实现，会错开
如果把标题变为inline元素，那么如果后面跟着一个图片，他就不会自动换行