Interoperable Web pages will function as intended in all User Agents; Graphical Browsers that display text and images such as MS Internet Explorer, Firefox, Google Chrome, Opera, Safari, Konqueror, et al. Textual Browsers such as Lynx that only display text; Screen Readers (such as JAWS) that read pages from left to right -- top to bottom -- and render the text as synthesized speech or transport it to a Braille reader; Mobile devices (such as Smart Phones); Search Engines such as Google -- that display textual information and hyperlinks
Pages should be checked in multiple Graphical Browsers at various screen resolutions to insure that they function as intended, which means that all content will display in the correct sequence, navigation will be as intended, and all hyperlinks will work correctly. I test all of my pages for correct function in current versions of, MSIE, Firefox, Google Chrome, Opera, Safari, et al. 1280 by 1024, 1024 by 768, 800 by 600 resolutions, and small screen rendering mode. I also test them for accessibility in the Lynx textual Browser and the TextAloud audio screen reader.
An important feature of Interoperable Web pages is the use of Valid Markup in their construction. In fact, the W3C includes the following as part of the successful validation output for their Markup Validator: "To show your readers that you have taken the care to create an interoperable Web page, you may display this icon on any page that validates ..."
Do not aspire to "pixel perfect" layouts. Refer to:
Emulating page function in textual Browsers and Screen Readers:
Fire Vox - A Screen Reading Extension for Firefox
This is a full featured audio Screen Reader that functions flawlessly as a Firefox Browser extension. It is accompanied by an excellent online manual and tutorials. The Fire Vox package (including download, installation and online manual) is available at the Fire Vox Information page.
Inexpensive audio Screen Reader:
TextAloud is easy to install, set-up, and use. Toolbars are installed for IE & Firefox Browsers. It is a Windows implementation and is available at NextUp.com as a 15 day free trial download.
Web authors do not know what type of User Agent, graphical Browser, textual Browser, or Screen Reader, each of their visitors will use to access their web pages, or what screen size (resolution) they have. They certainly have little control over how their web pages will be viewed and displayed. As the WWW progresses, Web authors will have less and less control over the way users view their web pages in graphical Browsers. Many users, especially People with Disabilities, will exert more and more control over how they view and navigate web pages. Newer graphical Browsers permit users to do such things, via drop-down menus, as:
A quick and handy way to zoom and/or change text size via keyboard shortcuts when viewing web pages:
When using MSIE .....
When using Firefox ..... same as with MSIE (You can specify "change text size only" in Firefox via the top menu bar: View ---> Zoom ---> Zoom Text only. Setting remains until changed. The above keyboard shortcuts will now only change the text size leaving the image sizes as original).
When using Safari/Chrome ..... same as with MSIE except the keyboard shortcuts only change the text size leaving the image sizes as original.
When using Opera ..... same as for Firefox/MSIE except use shift + for Zoom out.
The above procedures are especially well suited for web page viewing by People with Disabilities who cannot use a mouse (duchenne muscular dystrophy, etc.) and therefor rely on stylus/keyboard functions. Visitors who use a scrolling mouse can zoom/increase text size very fast via ctrl scroll -- for all Browsers listed above -- keyboard shortcut ctrl 0 (zero) returns to original size.
Web developers/authors might want to check pages they are composing to be sure navigation is not affected by incremental zooming (visitors will seldom zoom more than three increments). In my experience, many visitors (especially those with diminished vision) to web pages now increase the text size by one or two increments for easier reading especially when very small text is encountered.
Newer computer screen conformations and smaller viewing devices are necessitating more flexible/fluid Markup & CSS (style sheet) authoring these days. Of particular interest is the use of cell phones to browse web pages. The Opera Web pages, Opera Mobile Browser, and Small Screen Rendering Technology are excellent references, the latter one being especially valuable for Windows O/S Web authors.
Mobile/Small Screen Rendering:
Although some of the following principles are applicable to good Web page design in general, the tabulation includes considerations that are especially important for successful Web page rendering in Mobile/Small Screen devices with their narrow windows and columnar displays:
NOTE: the foregoing information relates to generating or changing "regular" Web pages to accommodate Mobile/Small Screen device rendering, or at least optimize their display in such devices, as much as possible. Designing specifically for Mobile/Small screen rendering requires different approaches and techniques. Two basic references are: Designing for the Mobile Web and W3C XHTML Basic 1.1
The Firefox Graphical Browser with Chris Pederick's Web Developer Extension installed is an outstanding tool for Web authors. The download is very easily installed and only a quick re-start of Firefox is needed to activate it.
The WAT IE Browser Accessibility Toolbar is an outstanding extension that is reminiscent of Chris Pederick's Web Developer Extension for the Firefox Browser.
The Developer Console offered by Opera also provides an excellent array of Web authoring tools.
Following are two invaluable Browser references -- be sure to read the caveats: