I am working on M2 and I've set M2 developer mode. My site home page works fine but the other pages are not loading. They just show errors and warnings on inner pages.
Check layout of specific files in your module or theme where this error is showing.
if you are using linux / ssh ( run this command it would find and list the file where this is added / declared )
grep -r -i --include \*.xml '<css' app/design/frontend/
Hope it Helps.
This is likely an error on the theme XML files.
<css> tag must only be located as child of the
I would suggest to run
grep command to locate all the files with the
<css> tag and look out for the tag's parent if its only under the
You may refer to
to check what elements should be under the