Negative length values are illegal.
As with most shorthand properties, you don’t need to specify all the properties listed, but any omitted properties will revert to their default values. In the case of {border-style:}, if you omit a value no border will show at all, because the default value is none. A {border:} will only be visible as long as the {border-style:} property has been set to something other than {none;} or {hidden;}, or has been restated explicitly after the initial shorthand declaration. Otherwise, no {border:} will show and the {border-width:} will be reset to {zero;}. Therefore, it’s good practice to specify a value for the border’s style when you’re using shorthand notation.
Specifies the border of a box. The value can combine {border-style:}, {border-width:} and {border-color:} for a whole four-sided border (with the {border:} property) or just one side of a {border:} (with the other four).
Sets or retrieves the properties to draw around the object.
The border property is a composite property that sets the width, style, and color values for all four sides of an object.
You must specify a style when specifying a width or color; otherwise, the {border:} does not render.
All individual border properties not set by the composite border property are set to their default values. For example, the default value for {border-width:} is {medium}.
The setting border=thin is identical to border=thin none; the default value for the border color is the same as the text color if one is not initially set. So, not only does the property set width to thin, it also clears any style or color values previously set.
Setting a border to zero or omitting the attribute causes no border to be displayed. Supplying the border attribute without a value defaults to a single border.