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March 19, 2015

Improving interoperability with DOM L3 XPath

As part of our ongoing focus on interoperability with the modern Web, we’ve been working on addressing an interoperability gap by writing an implementation of DOM L3 XPath in the Windows 10 Web platform. Today we’d like to share how we are closing this gap in Project Spartan’s new rendering engine with data from the modern Web.

Some History

Prior to IE’s support for DOM L3 Core and native XML documents in IE9, MSXML provided any XML handling and functionality to the Web as an ActiveX object. In addition to XMLHttpRequest, MSXML supported the XPath language through its own APIs, selectSingleNode and selectNodes. For applications based on and XML documents originating from MSXML, this works just fine. However, this doesn’t follow the W3C standards for interacting with XML documents or exposing XPath.

To accommodate a diversity of browsers, sites and libraries wrap XPath calls to switch to the right implementation. If you search for XPath examples or tutorials, you’ll immediately find results that check for IE-specific code to use MSXML for evaluating the query in a non-interoperable way:

In our new rendering engine, the script engine executes modern Web content, so a plugin-free, native implementation of XPath is required.

Evaluating the Options

We immediately began to cost the work to implement the entire feature. Our options included starting from scratch, integrating MSXML, or porting System.XML, but each of these was too costly for its own reasons. We decided to start implementing a subset of XPath while working on the full implementation at the same time.

In order to know what subset of the standard to support, we used an internal crawler that captured the queries used across hundreds of thousands of the most popular sites on the Web. We found buckets of queries that had the form

  • //element1/element2/element3
  • //element[@attribute=”value”]
  • .//*[contains(concat(” “, @class, ” “), ” classname “)]

Each of these queries maps cleanly to a CSS Selector that could return the same results that would be returned from our performant Selectors API. Specifically, the queries above can be converted to

  • element1 > element2 > element3
  • element[attribute=”value”]
  • *.classname

The first native implementation of XPath thus involved supporting queries that can be converted to a CSS selector and exposing the results through the DOM L3 XPath interface objects. Alongside this implementation, we added telemetry to measure the rate of success across broader Web usage, which accounted for the number of successful queries, number of failed queries, and the query string of the first failure.

Our telemetry from internal testing showed that 94% of queries successfully converted to selectors to unblock many Web sites. Of the failures reported through the telemetry, many took the form

  • //element[contains(@class, “className”)]
  • //element[contains(concat(” “, normalize-space(@class), ” “), ” className “)]

which can both be converted to the selector “element.className.” With these additional changes, the success rate improved to 97%, making our new engine ready for broader usage to support the modern Web.

Internal telemetry showing XPath query success rate over time in our testing
Internal telemetry showing Xpath query success rates

Addressing the Remainder

While this supports the vast majority of the Web, converting XPath queries to Selectors is inherently limited. Additionally, we were still left with the 3% of failing queries that require support of more of XPath’s grammar, such as functions, support for non-element and document nodes, and more complex predicates. Several sites mention various polyfills for XPath, including Mozilla Development Network, for platforms that do not provide adequate support for XPath. One of the polyfills commonly cited is wicked-good-xpath (WGX), which is an implementation of XPath written purely in JavaScript. Running WGX against our internal set of tests across the XPath spec showed 91% compatibility with competitor native implementations. The idea of using WGX as a fallback for the remaining 3% became a compelling bet because it was performant and our tests already measured its functionality against other implementations. Further, wicked-good-xpath is an open source project under the MIT license, and our usage of it would align with our goals to embrace and contribute back to code from the open source community. However, JavaScript had never been used within IE to build this kind of platform feature before.

In order to support WGX without polluting a Web page’s context, we created a separate, isolated script engine dedicated to WGX. With a few modifications to WGX that provide entry points for invoking functions and accessing results, we marshal the data from the page to the isolated engine and evaluate expressions with WGX. With WGX enabled to handle native XPath queries, we see immediate gains from sites missing content in our new engine rendering the modern Web.

Before: Available prices for products don't appear on After: Available prices for products appear on

Before and after: Available prices for products now appear correctly on

Before: Lotto numbers are not displayed After: Get the latest Lotto numbers

Before and after: Lottery numbers now correctly display where they were previously missing

While browsing the Web with WGX, we found a few bugs that exhibit behavior which diverges from other browsers’ implementations as well as the W3C spec. We plan to make contributions back to the project that make WGX more interoperable with native implementations.

Our new rendering engine now has support for XPath to run the modern Web to enable more functionality and show more content for our customers. With data-driven development from the modern Web, we landed on a data-driven outcome of a low-cost and performant implementation on existing technologies and standards as well as open source code. Grab the latest flight of the Windows 10 Technical Preview to try for yourself! You can reach us with feedback via the Internet Explorer Platform Suggestion Box on UserVoice, @IEDevChat on Twitter, and in the comments below.

– Thomas Moore, Software Engineer, Microsoft Edge