  The HTML5 <canvas> is widely used to display high quality graphics in web
applications. However, the combination of web, GUI, and visual techniques that
are required to build <canvas> applications, together with the lack of testing
and debugging tools, makes developing such applications very challenging. To
help direct future research on testing <canvas> applications, in this paper we
present a taxonomy of testable <canvas> issues. First, we extracted 2,403
<canvas>-related issue reports from 123 open-source GitHub projects that use
the HTML5 <canvas>. Second, we constructed our taxonomy by manually classifying
a random sample of 332 issue reports. Our manual classification identified five
broad categories of testable <canvas> issues, such as Visual and Performance
issues. We found that Visual issues are the most frequent (35%), while
Performance issues are relatively infrequent (5%). We also found that many
testable <canvas> issues that present themselves visually on the <canvas> are
actually caused by other components of the web application. Our taxonomy of
testable <canvas> issues can be used to steer future research into <canvas>
issues and testing.
