Like all tags, <img> tag has attributes. For example, border is one of the attributes of the <img> tag. One can place a border of varying size around an image, as shown here:
As you see, the border width varies depending upon the value of the attribute. The value must be a number and the unit of measure is pixels -- it specifies the width of the border in pixels. If you omit the border attribute, the default value is 0.
