The height CSS property specifies the height of the content area of an element. The content area is inside the padding, border, and margin of the element.
Percentages The percentage is calculated with respect to the height of the generated box's containing block. If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to auto. A percentage height on the root element is relative to the initial containing block.
Computed value a percentage or auto or the absolute length
Animatable yes, as a length, percentage or calc(); when both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers.
Canonical order the unique non-ambiguous order defined by the formal grammar
table { height: 100%; }
img { height: 200px; }
form { height: auto; }
/* make sure percentage height of body's children works as expected */
html, body { margin: 0; height: 100%; }