Safari hasn't actually been particular far behind implementing industry standards. As far as I can tell, it's more that people seem to believe that Google dictates industry standards and base everything on when Chrome supports it as opposed to when it actually gets standardized.
SSE's
W3C draft standard in 2012. Supported in Safari in 2010.
web sockets
This one is true. IETF standard 2011. Supported fully in Safari 2013.
IndexedDB API
W3C recommended standard in 2015. Supported in Safari in 2014.
animations
If we're talking the Web Animations API, it hasn't been standardized yet (W3C working draft) and level 2 isn't even that far.
relative color syntax
Not standardized yet. It's currently a W3C working draft.
container queries
Not standardized yet. It's currently a W3C working draft.
a bunch of <video> stuff
Need specifics.
flexbox
W3C candidate recommendation 2018. Supported in Safari 2013.
SSE's
W3C draft standard in 2012. Supported in Safari in 2010.
web sockets
This one is true. IETF standard 2011. Supported fully in Safari 2013.
IndexedDB API
W3C recommended standard in 2015. Supported in Safari in 2014.
animations
If we're talking the Web Animations API, it hasn't been standardized yet (W3C working draft) and level 2 isn't even that far.
relative color syntax
Not standardized yet. It's currently a W3C working draft.
container queries
Not standardized yet. It's currently a W3C working draft.
a bunch of <video> stuff
Need specifics.
flexbox
W3C candidate recommendation 2018. Supported in Safari 2013.