1. Scope of use
1. Inline elements, inline-block elements and table-cell elements are valid, but invalid for block-level elements
2. Attribute value
Negative numbers, percentages and keywords
3. The principle of action
1. Used to define the vertical alignment of "surrounding text, inline elements, and inline-block elements" relative to the baseline of the defined vertical-align element.
2. The vertical-align attribute of the table-cell element is for itself. The alignment of inner child elements defined by vertical-align relative to itself.