Indicates the horizontal dimension for the resulting image display. If necessary, the image is scaled to the specified size. The value of this attribute is a real number optionally followed by a unit of measure from the set of pc, pt, px, in, cm, mm, em (picas, points, pixels, inches, centimeters, millimeters, and ems respectively). The default unit is px (pixels). Possible values include: "5", "5in", and "10.5cm". If a width value is specified and no height value is specified, the height will be scaled by the same factor as the width. If both a height value and width value are specified, some implementations may not be able to scale the two directions by a different factor and may therefore ignore one of the two values.