Hi There,
This is indeed a tricky error to debug. One solution I have found to work if that suggested on the crib sheet PsychoPy Python to Javascript crib sheet - Google Docs ).
The step by step stages for implementing that are:
a. Experiment settings> export html > set as manual
b. make any edits to builder file (e.g. adding components setting text size etc.)
c. File > export html
d. open up index.html in text editor
e. replace pixi in html (using the lines specified in the crib sheet)
f. sync s usual and run in browser
Hope this helps,
Becca