1, a block element may comprise some inline elements or block elements, but the elements can not contain block inline element can contain inline elements.
<div><h1></h1><p></p></div> 正确
<a href="#"><span></span></a> 正确
<span><div></div></span> 错误
2, not on the block-level elements <p> inside
<p><ol><li></li></ol></p> 错误
<p><div></div></p> 错误
3, there are several special block element can contain inline element can not contain block-level elements. These special labels are h1 ~ h6, p, dt
4, the div tag may comprise li
<li><div></div></li>
5, block-level elements in parallel with block elements, inline elements and inline elements in parallel
<div><h2></h2><p></p></div> 正确
<div><a href="#"></a><span></span></div> 正确
<div><h2></h2><span></span></div> 错误