Abstract
Ensuring the correct presentation and execution of web sites is a major concern for system developers and administrators. Unfortunately, only end users can determine which resources are available and working properly. For example, some internal or external addresses might be unavailable or unreachable for specific clients, while seemingly available resources, like JavaScript, might run with errors in some browsers. While standard monitoring and analytic tools certainly provide valuable information on web pages, problems might still escape such measures, to reach end web users. To demonstrate the limitations of current tools, we ran an experiment to count web page errors in a sample of 3,000 web sites, including network and JavaScript errors. Our results are significant: as many as 16% of the top 1,000 sites have errors in their own resources; less popular sites have even more. Based on these results, we make a review of three client-side monitoring approaches to mitigate such errors: stand-alone applications, browser extensions and JavaScript snippets with analytic tools. Interestingly, even the latter approach, which requires no software installation, and involves no security changes, can cover a large fraction of existing web errors.