Okay I have a few:
NICE is not compatible with HTML comments … so avoid comments <!-- comment --> Screenshot by Lightshot
In general … you have to be super careful with your html … Below are some examples pointed out by support to me that may help others … (hard to find but obvious once pointed out ) …
With attribute construct errors - it generally means something is wrong with your html
things like: classes shouldn’t start with space Screenshot by Lightshot
invalid HTML Screenshot by Lightshot, ul shouldn’t be in the tag p Screenshot by Lightshot
with attribute name redefined error the solution for me was that the input in form shouldn’t include two “names” (Screenshot by Lightshot)
and finally the most important tip from support: double-check your code (the code editor in the admin shows where the code is invalid and the error message indicates the line with the error)