>Students can't check their spellings in a a label or a webpage
Sorry for my delayed response. The code is intended for Moodle users with editing privileges. My suggestion about the iframe was to add the code to a label you create and that students have access to. You set it up using your role as teacher. Students use it to check their text.
I also noticed that it has trouble with apostrophes and wants to escape them. Perhaps, this issue will be resolved. There is an email address for suggested improvements.
I am tempted to say that this limitation doesn't matter as the goal seems to provide corrective feedback, not to spit back error-free text--a pedagogical advantage to my mind since it provides the location of the error, the nature of the error, and a suggestion for correcting it. The job of actually correcting errors remains with the learner. With Word, when it catches an error, learners can simply right-click and the correction is automatic.
>I couldn't get it to highlight my deliberate errors
What were your deliberate errors? I am eager to see the limitations of this resource, both in terms of language pedagogy and usability.