HTML and CSS Reference
In modern browsers it works fine. However, when validating as CSS 2.1, the validator would give errors. Even in
CSS3, only the first one ( opacity ) is valid, which works in Firefox, Opera, and Safari but not in IE (which requires the
filter property). Thus, validity can be obtained by removing the last three properties; however, the result won't work
under Internet Explorer and older versions of other browsers.
A valid, cross-browser solution could be the transparent background image such as the one shown in Listing 15-24.
Listing 15-24. A Transparent PNG Background File Is a Robust Solution
Not all documents can be checked by validators. Incorrect serving or temporary server errors are the most common
reasons. Without providing data required for automatic document type detection or character encoding detection,
validation cannot be performed or provides unreliable results.
Common News Feed Errors
Valid news feed channels should be well-formed and meet all general XML criteria. Even so, some errors might occur
in manually updated news feeds because of human error. One of the most common errors is an incorrect date. If the
publication date is earlier than the date of updating, validators give an error. This is referred to as an implausible date .
Care must be taken to apply the proper offset whose misuse can lead to the same problem.
Common Script Errors
Scripts are out of the scope of markup validators. Consequently, extended care must be taken to ensure correctness
and proper functionality. It should be ensured that a web page remains usable even if the scripts cannot be executed
for whatever reason. It is important to keep in mind that alternate content written for scripts cannot be checked by
software tools, and their evaluation depends on human decision.
Common Accessibility Errors
In contrast to markup or CSS errors where errors depend on the language version being used, accessibility errors are
determined by the version and level of guidelines considered. A comprehensive overview of accessibility errors and
their solutions (“Failures of Success Criteria”) is collected in the W3C group note describing techniques and failures
for WCAG 2.0 . The most common accessibility errors can be summarized as follows:
Lack of structured markup or table layout
Images that convey important information are embedded through CSS
User control is missing to stop or pause blinking, scrolling, and automatically played sound
files or videos