In a recent Q&A on Youtube, the GTM product manager said two things which made me consider the current recommendation of putting the GTM script at the beginning of the body tag.
https://youtu.be/XBCNJo9qGH8?t=6m23s
The video highlights the positives of having it early, and that the drawbacks are HTML validation. Synchronous tags will be supported by the end of the year, and I hope to future-proof with that in mind (for synchronous A/B scripts)
It seems desirable to be able to deploy A/B snippets from Google Analytics and optimizely through GTM.(it´s very desirable because I don´t want a synchronous script being there for several months).
What are other negative consequences of putting the script in the head section?