Epubcheck 4 is in the horizon, you should take another look at your books.
ePubcheck 4, the IDPF validator is currently in Alpha and available for download if you want test it. I downloaded it and made a first pass to what I thought was a valid book and was, not so pleasantly, surprised at how much it has changed.
I say not so pleasantly surprised because of the additonal breadth of checks the tools performs and the cryptic nature of the messages has prompted me to rethink the way in which my documents are laid out, particularly considering that I use Docbook to generate my epub content and it’s not laid out as standard HTML (uses tables to layout content wich means the table doesn’t need the same structure needed for data tables… I know… so 1990s but I did not create the stylesheets).
I’m concerned that publishers will use the new litmus test of epubcheck 4 to reject books taht would have normally passed the epubcheck 3.0.1 validation test.
The positive side of all these warnings is that documents will be more accessible because of the additional checks, assuming that authors (and publishers) bother with implementing any of the rules their books are flagged for.
For those interested, the full list of epubcheck 4 messages is listed below:
ID Severity Message Suggestion ACC-001 WARNING Img and Area html elements should include an 'alt' attribute for accessibility. ACC-002 WARNING Input html elements should be referenced by a corresponding label element. ACC-003 WARNING Non-text html elements should include a 'title' attribute for accessibility. ACC-004 WARNING Html 'a' element must have text. ACC-005 WARNING Table heading cells should be identified by 'th' elements for accessibility. ACC-006 USAGE Tables should include a 'thead' element for accessibility. ACC-007 USAGE Epub should use epub:type attributes. ACC-008 USAGE Epub should have 1 landmarks nav element. ACC-009 USAGE MathML should either have an alt text attribute or annotation-xml child element. ACC-010 USAGE Headings should not be used within blockquote and figure elements. ACC-011 USAGE Link elements within SVG should include an xlink:title attribute. ACC-012 USAGE Table elements should include a caption element. ACC-013 WARNING Content file contains at least one inline style declaration. Inline styles are not compatible with accessibility settings and display personalization. Use CSS Styles instead. ACC-014 WARNING CSS Selector font-size attribute value does not use a relative size. Acceptable values include percentages, em multiples, 'larger', 'smaller', 'normal' or 'inherit'. ACC-015 WARNING CSS Selector font line-height attribute value does not use a relative size. Acceptable values include percentages, numeric multiples, em multiples, 'normal', or 'inherit'. ACC-016 USAGE CSS Selector font-size attribute value should use a relative size. Acceptable values include percentages, em multiples, 'larger', 'smaller', 'normal' or 'inherit'. ACC-017 USAGE CSS Selector font line-height attribute value does not use a relative size. Acceptable values include percentages, numeric multiples, em multiples, 'normal', or 'inherit'. CHK-001 ERROR The custom message overrides file was not found. CHK-002 ERROR Unrecognized custom message id %1$s encountered in message overrides file '%2$s'. CHK-003 ERROR Unrecognized custom message severity '%1$s' encountered in message overrides file '%2$s'. CHK-004 ERROR The custom message contains too many parameters in message overrides file '%1$s'. CHK-005 ERROR The custom suggestion contains too many parameters in message overrides file '%1$s'. CHK-006 ERROR Unable to parse the custom format parameter in message overrides file '%1$s'. CHK-007 ERROR Error encountered while processing custom message file '%1$s': "%2$s". CSS-001 ERROR The '%1$s' property must not be included in an ePub Style Sheet. CSS-002 ERROR Empty or NULL reference found. CSS-003 ERROR Only UTF-8 and UTF-16 encodings are allowed, detected %1$s. CSS-004 ERROR Only UTF-8 and UTF-16 encodings are allowed, detected %1$s BOM. CSS-005 ERROR Conflicting alternate style attributes found: %1$s. CSS-006 WARNING CSS position:fixed property is not allowed in ePub v3 documents. CSS-007 WARNING Font-face reference %1$s refers to non-standard font type %2$s. CSS-008 ERROR An error occurred while parsing the CSS: %1$s. CSS-009 USAGE Use of certain CSS such as Columns, Transforms, Transitions, box-sizing or KeyFrames can cause pagination issues. CSS-010 WARNING Use of non-standard stylesheet. CSS-011 USAGE Excessive number of css files. Consider merging CSS files to reduce the number of CSS files. CSS-012 USAGE Document links to multiple CSS files. CSS-013 USAGE CSS Selector attribute is declared !Important. CSS-015 WARNING Alternate Stylesheet has no title. CSS-016 WARNING Alternate Stylesheet precedes primary stylesheet. CSS-017 WARNING CSS Selector specifies absolute position. CSS-019 WARNING CSS font-face declaration has no attributes. CSS-020 WARNING CSS font selector declaration uses unexpected font-size value '%1$s'. Acceptable values include percentages, em multiples, 'larger', 'smaller', 'normal' or 'inherit'. CSS-021 WARNING CSS Selector font shorthand specifies an invalid System Font. Acceptable values include 'caption', 'icon', 'menu', 'message-box', 'small-caption', 'status-bar', or 'inherit'. CSS-022 WARNING CSS Selector specifies global margin setting. CSS-023 USAGE CSS Selector specifies media query. CSS-024 USAGE CSS class Selector is not used. Remove unused CSS selectors. CSS-025 USAGE CSS class Selector is not found. Check for typos or define a class selector to document the use of the class. CSS-027 USAGE CSS Selector specifies absolute position. CSS-028 USAGE Use of Font-face declaration. HTM-001 ERROR Any publication resource that is an XML-based media type must be a valid XML 1.0 document. XML version found: %1$s. HTM-002 WARNING The installed xml parser doesn't support xml version verification. Xml files must be a valid XML 1.0 document. HTM-003 ERROR External entities are not allowed in ePub v3 documents. External entity declaration found: %1$s. HTM-004 ERROR Irregular DOCTYPE: found '%1$s', expected '%2$s'. HTM-005 USAGE An external reference was found. HTM-006 USAGE XHTML Named Entities are in use. Consider using numbered entities instead. HTM-007 WARNING Empty or whitespace-only value of attribute ssml:ph. HTM-008 ERROR The src attribute is required. HTM-009 ERROR The DOCTYPE provided is obsolete or irregular and can be removed. HTM-011 ERROR Entity is undeclared. Define the entity or use the numbered entity instead. HTM-012 WARNING Found a link to a CFI in an external book. HTM-013 USAGE Intra-Publication CFIs found in document. HTM-014 WARNING Invalid file extension for HTML5 file, expecting (html, htm or xhtml). HTM-015 WARNING HTML4 DOCTYPE definition within ePub v3. HTM-016 WARNING HTML5 DOCTYPE definition within ePub v2. HTM-017 ERROR Content file has different language value in attributes xml:lang and lang. HTM-018 ERROR Content file has invalid language value at attribute xml:lang. HTM-019 ERROR Content file has invalid language definition at attribute lang. HTM-020 USAGE Content file doesn't contain xml:lang attribute. HTM-021 USAGE Content file doesn't contain lang attribute. HTM-022 USAGE Document contains unnecessary DIV or SPAN tags. Merge DIV or SPAN tags when they are consecutive and use the same style. HTM-025 WARNING Non-registered URI scheme type found in href. HTM-027 WARNING Lists should include 2 or more list items. Lists should have more than one item for accessibility. HTM-028 WARNING Input html elements should include an id. HTM-029 WARNING Label html elements should include a 'for' attribute referencing the id of an input element. HTM-033 WARNING Html head element does not have a title. HTM-036 WARNING IFrames are highly discouraged. HTM-038 USAGE Ensure b, i, em, and strong elements are used in compliance with W3C HTML5 directives. CSS styles are usually more appropriate for italics or bold text. HTM-043 USAGE SVG elements should include an xml:lang and lang attributes. HTM-044 USAGE Namespace uri '%1$s' was included but not used. HTM-045 INFO Encountered empty href. HTM-046 ERROR Fixed format item has no viewport defined. HTM-047 ERROR Html viewport is missing height and/or width. HTM-048 ERROR SVG ViewBox is missing on fixed format document. MED-001 ERROR Video poster must have core media image type. MED-002 ERROR %1$s element doesn't provide fallback. MED-003 ERROR Non-standard image resource of type %1$s found. MED-004 ERROR Image file header may be corrupted. MED-005 ERROR Media Overlay audio reference %1$s to non-standard audio type %2$s found. MED_006 WARNING Some browsers do not support rendering SVG images which use a filename in the xlink:href property. NAV-001 ERROR The nav file is not supported for ePub v2. NCX-002 ERROR