Missing Title

The website must contain a precise "title tag", a page title. This is based on the following:

• The title is displayed on the title bar of the browser window.
• The title is used when setting a bookmark (or favourite).
• The file title is shown in the browser in the list of pages previously searched.
• Many search engines offer the title as a hyperlink.
• Search engines use the content of the title for their search.
• Website users often only read the first words and should therefore have enough information about the website.

Suboptimal Target

When clicking on a link, web users expect to see the new page in the existing window. Most users lose control if there are several windows open, are unable to navigate back with the back function and get lost. At the same time, and this is particularly true for old computers, the system can get overloaded and crash if too many browser windows open. If new windows open but are not seen by the user, the perception is that the website does not work properly. Experienced users who want to open new windows know how to do this by using the function "Open (link) in New Window", or with similar comands.

There is one important exception to this rule: files in .PDF or other Office format can be opened in new windows. Users know to close them to return to the original window. In this case, users should be informed that a new window opened, and they should be given the opportunity to save the file.

Technical hint: For XHTML Strict in particular, there is the possibility to send a .PDF file with ContentType "application/pdf" and HTTP Header content-disposition: attachment; filename=filename.pdf, since there is no target attribute. This test does not take it into account yet, but this is a feasible solution.

Source and further information:
"Prioritizing Web Usability" by Jakob Nielsen/Hoa Loranger

Image with Missing Alt Text

Alternative text is displayed if an image cannot be downloaded or if the mouse pointer is moved over the image*. The alternative text is on one hand important for visually impaired visitors, using an audio tool to read the content of the website and the alternative text of images.
On the other hand, alternative text can also be recognised by search engines. Alternative text needs to be phrased in a meaningful manner, though it should not be a detailed description of the image.

Further information:
http://www.bjoernsworld.de/html/alt-text.html
http://webdesign.crissov.de/alt-Text#Start

* Internet Explorer shows alternative text as a tooltip if the mouse pointer is rested on the image. This is not prohibited but it can annoy page authors. It does also indicate that the alt-attribute refers to this tooltip, which is wrong. Should you want to add a descriptive tooltip to your images, make use of the attribute "title".
Quelle: http://de.selfhtml.org/html/grafiken/einbinden.htm#referenz