One might have been into a difficult situation while implementing Adobe Target on their site page where everything (setup/configuration etc.) looks perfect following the best practices but felt that Target library is not loading.
You get to see below warnings while doing debugging.
"AT: Adobe Target content delivery is disabled. Update your DOCTYPE to support Standards mode."
"AT: Target library is either not loaded or disabled, no request will be executed"
The solution to this problem is the support required for standard mode on your web pages. In simple words, your page demands the information for your browser that what document type to expect'. Thus, adding an information as '<!DOCTYPE html>' on your web page should do the job from my experience.