These values specify the maximum field size in the returned image in RA and Declination in degrees. The image can be rotated relative to the input image by the amount specified here in degrees. The rotation is in the sense that north rotates towards east. The returned image may be one of several types. This may not be completely adequate in all cases and use of an external viewer with some control over brightness and contrast is recommended.
FITS files contain the full range of values in the correct physical units.
Web browsers use external viewers to display files like FITS images which they do not understand. In order to invoke an external viewer, it is necessary to associate a particular viewer with a particular MIME type. At very least, control over the contrast and brightness is needed if not some control over the range of values displayed in order to get the best display of the objects in the image. One suitable external viewer is the FITSview family of FITS image viewers is available for a variety of computer systems. In order to use an external viewer with your Web browser you need to install the software on your computer.