I followed the John Nelson video below to create symbology for trees on an antique map:
https://www.youtube.com/watch?v=sy3Rerl05yg
This looked great on ArcGIS Pro, but I was unable to find a way to use this symbology in ArcGIS Online. When I attempted to share the layer as a web layer the result had no symbology; the map was completely blank. Do you have any suggestions for creating a similar symbology in ArcGIS Online?
You might look at using a vector polygon fill for your tree polygon layer. It is not the same, but you can get a good effect.
I would try enabling the 'Use symbol types compatible with all clients' check box and see if that makes a difference (located just below Item Details, under Select a configuration).
Otherwise, as suggested above, use common ESRI project style symbols and you shouldn't have an issue.
@AndrewBarton I would recommend using the suggestion by @Debert
I have attempted this suggestion using the same style and this has worked for me, just to note at 3.4 ArcGIS Pro the location of this setting has moved from the above screenshot
It is now under the configuration tab as seen below.