While the precision and reliability of response time measurements performed through web-based interfaces has been evaluated, sequences of keystrokes have never been investigated in detail. Keystrokes often occur in much more rapid succession than response times, and operating systems may interpret successive or concomitant keyboard events according to automatic and user-based settings. Sequence keystroke timing could thus be more sensitive than single response times to noise in on-line measurements. Here, we quantified the precision and reliability of timing measures performed during sequences of keystrokes. We used the JavaScript jsPsych library to create an experiment involving finger- movement sequences and run it on-line on 633 participants. We manipulated the structure of three keystrokes motor sequences, targeting a replication of previous findings regarding response times (RT) and inter-keystroke intervals (IKIs). Our on-line data accurately reproduced the original results, and allowed the novel assessment of demographic variables such as age or gender. In parallel, we also measured the objective timing accuracy of the jsPsych interface using specialized hardware and software, showing a constant 60 ms delay for RTs and a 0 ms delay for IKIs across the sequences. The distribution of IKIs revealed quantizing for a majority of participants, most likely due to the sampling frequency of the USB keyboards. Overall, these findings indicate that JsPsych provides good reliability and accuracy in sequence keystroke timing for mental chronometry purposes, through on-line recordings.