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Ready.mobi: A free tool to test your mobile site
ready.mobi (http://ready.mobi/launch.jsp?locale=en_EN) is a really good free tool developed by the dotMobi company  based on W3C best practice . It allows to test a page, and getting a score indicating how well it will work with mobile, and detailed advices how to improve the site, download time and cost estimate, visualising the page in emulators. That tool greatly helped the helsinki.mobi site to be up and running in 2 weeks.
But this tool has its limitations. The tool is based on W3C best practices that give guidance how to build web sites working with most mobile phone currently on the market. It assumes low capabilities of the mobile browser. A top score on the ready.mobi tool means that the site should work fine with most of the phones. But it also means that the page do not take advantage of all the capability of a recent S40 or S60 browsers.
The tool is still useful to test pages tailored for S60, but the developer just needs to interpret the results. For example, the tool will give warnings if the page exceed 20kB (based W3C Default Delivery Context assumption), or if scripting is used. However, such features are likely to bring a better experience on S60 devices.
In conclusion, the ready.mobi tool is very convenient for mobile developers. But developing a mobile site still requires a good understanding of the type of devices targeted.
And by the way, to test the developer, there is the dotMobi mobile web developer certification . If you are a mobile developer and you want to prove your skills this might be useful for you. Note that, despite the name, the dotMobi Developer Certification is based primarily on W3C standards in general, rather than dotMobi's rules in particular. There are some sample questions listed on their site: