Apologies for the slow response. I had temporarily stopped working on the project. It was my mistake. I had implemented a modified version of the HTML integration that is suggested on the languagetool site. I had modified the form with class .checkform and moved the submit button outside of it, so the error document.checkform._action_checkText was expected as ._action_checkText only exists outside of the form. I have since rebuilt the application with the submit button contained inside the form, so it all works fine, but changing the line to document._action_checkText should be a start, then