XML data that conforms to the rules of XML syntax. There is typically little or no tolerance for straying from any electronic standard. However, HTML, the tag-based format language created prior to XML, was allowed to meander. Web browsers were written to tolerate sloppy HTML coding, resulting in variations of HTML pages that caused problems down the line. When XML came along, adherence to the rules was enforced, hence, "well-formed" code. See XML.