Following news last week that the Firefox browser now has 11 percent of the UK market, experts told web developers to ensure sites work with open-source browsers as well as Internet Explorer, or risk alienating potential customers who cannot fully access sites.

Deri Jones, chief executive of web testing company SciVisum, told IT Week that as many as one in 10 sites still fail to provide access to Firefox users, even though it is used by 11 percent of UK web surfers, according to figures from French metrics company XiTi.

Jones advised web developers to develop code around the W3C’s Cascading Style Sheets 2 specification, which simplifies site development by separating content and presentation, and makes sites more accessible to disabled people.

“Those that stick to standards have much more reliable, stable and fast web sites,” Jones argued. “They should keep it simple, and [taking into consideration] accessibility issues also improves the performance of web sites.”

Mike Davis of analyst firm Butler Group said that designers who neglect open-source compatibility are being lazy and putting their firms at a disadvantage. “Web sites are trying to publicise an organisation or get people to undertake trade so the sites should [be universally accessible],” he argued.

However, Gary Barnett of analyst company Ovum cautioned that despite its growing market share, Firefox is still virtually unknown to many types of user.

“My advice is to begin designing in a wholly standards-compliant way and then make a business decision whether to exploit the non-standard features in Internet Explorer and optimise the site [for IE],” he said. “But you must know if you do that it will diminish the experience for non-IE users when they come to the site.”

Barnett added that testing is also essential. “If you know [a certain] percentage of users will be using a browser and you don’t test for compatibility, your competence as a developer is in question,” he warned.

Zamir Cajee of web design company UKWebco said that plug-ins, Flash and data feeds in particular can cause compatibility difficulties for developers, and more problems may lie ahead when Microsoft releases a raft of new features with Internet Explorer 7 later this year.

Meanwhile, Mozilla is set to release a new version of Firefox this March for use with the Intel version of Apple’s Mac OS X, but it may struggle to displace the Safari browser which is popular with Mac users, according to reports.