Google recently posted an article on their testing blog: Survival techniques for acceptance tests of web applications (Part 1). In it they note the difference between various expressions used with Selenium to get access to an object on a web page:

For each method, we need to work out how to implement it in code. How could an automated test select the compose message icon? Do alternative ways exist? An understanding of HTML, CSS, and JavaScript will help you if you plan to use browser automation tools. All the visible elements of a web application are reflected in the Document Object Model (DOM) in HTML, and they can be addressed in various ways: the directions from the root of the document to the element using xpath; unique identifiers; or characteristics possessed by the elements, such as class names, attributes, or link text. Some examples of these addressing options are shown in the Navigation Options illustration below. (Notes: navigation using xpath is much slower than using IDs; and IDs should be unique.)

We agree that IDs are usually the best approach, but sometimes (especially with AJAX.NET applications) those IDs are actually really bad to use. The reason is that some frameworks (AJAX.NET, ExtJS, etc) use ID generation techniques that aren’t the same for every page view or every web app build.

As such, keep an eye out for suspicious IDs and be prepared to switch back to XPath or DOM locators when necessary. For example, if you see an ID like ctl00_ctl00_cp_cpTab_txtFirstName (generated from AJAX.NET) consider changing it to an XPath such as //input[contains(@id, 'txtFirstName')].

Related to that topic, they also note that scripts recorded in Selenium IDE should not be considered “done” and will likely need to be optimized and tweaked before they can be considered stable:

The open-source test automation tool Selenium (http://seleniumhq.org/) includes a simple IDE record and playback tool that runs in the Firefox browser. Recorded scripts can help bootstrap your automated tests. However, don’t be tempted to consider the recorded scripts as automated tests: they’re unlikely to be useful for long. Instead, plan to design and implement your test code properly, using good software design techniques.

The author goes on to suggest Firebug as a great tool for doing this kind of work, which we tend to agree with and have covered previously. But they missed one of the most important functions: the ability to easily test XPath expressions from within Firebug!

Very few Firebug users know about this, so take note: Firebug adds a $x function that can be called from the console. It takes an XPath in the form of a String and will spit out the HTML elements that it evaluates to directly back in the console. You can then mouse over the elements and check that the XPath is working correctly:

Armed with Selenium IDE, Firebug, and $x function, you should be able to build out solid Selenium scripts in no time!

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