HTML validation error: Non-space characters found before DOCTYPE
I found the root of problem. The general rule is:
If any(absolutely any!) file that take part in construction of the code of final HTML-page(the one to be sended to client) has encoding with BOM - final HTML-page WILL BE UTF-8-BOM. That is: you whole site should NOT contain even 1 file with BOM.
In my case I have total 1.3K files that make up my site. Only 4 files was BOMed:
- wp-config.php (in root of site)
- jquery.query.js (in include folder)
- cyr-to-lat.php (in plug-in folder)
- footer.php (in theme root folder)
And I was forced to re-save every and all of these 4 files as "UFT-8 without BOM" to get rid of "Non-space characters" validation error. When I did this (re-save files) - error is gone.