ArcGIS Online's built-in symbology is...not great. ArcGIS Pro allows you to use SVG files for your layer symbology, but when the publishing process rasterizes the symbol making resizing the symbol post-publishing pretty terrible.
If Esri instead supported the use of SVG symbols natively in ArcGIS Online and Portal, this would not be a problem at all. Esri should also convert all of their built-in symbology to SVG instead of PNG. the only place for a raster in a webmap these days is satellite imagery.