Property Reference - CSS Reference - page 220

Name
max-width
Values:
<length> | <percentage> | none
Initial value:
none
Applies to:
All elements except inline nonreplaced elements and table elements
Inherited:
No
Percentages:
Refer to the height of the containing block
Computed value:
For percentages, as declared; for length values, the absolute length; otherwise, none
Description:
Defines a maximum constraint on the width of the element (the exact nature of that width is
dependent on the value of box-sizing ). Thus, the element can be narrower than the declared
value but not wider. Negative values are not permitted.
Example:
#sidebar img {width: 50px; max-width: 100%;}