We just upgraded a development site from Magento 2.1.X to 2.2.
I have a custom CSS file called in the default_head_blocks.xml of my specific theme (which had no issues working at all with 2.1.X). The file was called in like this...
<css src="css/tires-custom.css" after="-" />
After our upgrade to 2.2 it now gives the following error...
1 exception(s): Exception #0 (Magento\Framework\Config\Dom\ValidationException): Element 'css', attribute 'after': The attribute 'after' is not allowed. Line: 31
I changed nothing in the file but not get the error. I don't understand why it would cause an error simply due to the upgrade.
If I remove the attribute
after="-" the site loads juts fine. The problem is I need to have my custom CSS loaded last.