I think this might also refer to the case when fonts are used (through CSS inside an svg which is than embedded into an image () tag.

In this case only embedded fonts will work (encoded w/ base64):

<style><![CDATA[@font-face {font-family: 'eEgji1HZxKV1:::Concert One';font-style: normal;font-weight: 400;src: url(data:font/ttf;charset=utf-8;base64,AAEAAAAOAIAAAwBgT1MvMpYAauMAAAIIAAAAYGNtYXABpQJnAAACaAAAAHRjdnQgAD8H1AAAAQgAAAASZnBnbZJB2voAAALcAAABYWdhc3AAAAAQAAAA7AAAAAhnbHlmd/DpXgAABqgAAAwwaGVhZBNSMSsAAAHQAAAANmhoZWEOGgUGAAABeAAAACRobXR4M+YFTgAAAZwAAAA0bG9jYRLRFmkAAAEcAAAAHG1heHACIAMmAAABOAAAACBuYW1lL+NOFwAABEAAAAJocG9zdP9tAGQAAAFYAAAAIHByZXAXLpg/AAAA9AAAABEAAQAB//8AD7AAKwCyAQQHK7AAIEV9aRhEAAAAACoAAAAI/nAAAAPsAAQFeAAIAAAAAABPAE8A9QE/AdkCegMxBBoElQUKBW4FpQYYAAEAAAANAIwABQA/AAIAAQAAAAAACgAAAgACWQABAAEAAwAAAAAAAP9qAGQAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAAH1f5mAAAGuP/I/9oGegABAAAAAAAAAAAAAAAAAAAADQUYAO4DHgAABFAAeAPWAAoGggAYA/wAVAP8AFwEEgBcBBIAegJyAJoD9gCWAkoAjAQ6AIQAAQAAAAEBBl4m3lRfDzz1AB0IAAAAAADLYfyZAAAAAOJ86Hn/yP5wBnoHpgAAAAgAAgAAAAAAAAADA/UBkAAFAAAFmgUzAAABHwWaBTMAAAPRAGYCAAAAAAAAAAAAAAAAAIAAACcAAABDAAAAAAAAAABweXJzAEAAIABuB9X+ZgAAB9UBmiAAARFAAAAAA+wFeAAAACAAAQAAAAIAAAADAAAAFAADAAEAAAAUAAQAYAAAABQAEAADAAQAIABCAFQAVwBhAGUAaQBsAG7//wAAACAAQgBUAFcAYQBlAGcAawBu////4f/A/6//rf+k/6H/oP+f/54AAQAAAAAAAAAAAAAAAAAAAAAAAAAAsAAsS7AJUFixAQGOWbgB/4WwRB2xCQNfXi2wASwgIEVpRLABYC2wAiywASohLbADLCBGsAMlRlJYI1kgiiCKSWSKIEYgaGFksAQlRiBoYWRSWCNlilkvILAAU1hpILAAVFghsEBZG2kgsABUWCGwQGVZWTotsAQsIEawBCVGUlgjilkgRiBqYWSwBCVGIGphZFJYI4pZL/0tsAUsSyCwAyZQWFFYsIBEG7BARFkbISEgRbDAUFiwwEQbIVlZLbAGLCAgRWlEsAFgICBFfWkYRLABYC2wByywBiotsAgsSyCwAyZTWLBAG7AAWYqKILADJlNYIyGwgIqKG4ojWSCwAyZTWCMhsMCKihuKI1kgsAMmU1gjIbgBAIqKG4ojWSCwAyZTWCMhuAFAioobiiNZILADJlNYsAMlRbgBgFBYIyG4AYAjIRuwAyVFIyEjIVkbIVlELbAJLEtTWEVEGyEhWS0AAAAAAAAIAGYAAwABBAkAAAFAAMIAAwABBAkAAQAWAKwAAwABBAkAAgAOAJ4AAwABBAkAAwAsAHIAAwABBAkABAAWAKwAAwABBAkABQAaAFgAAwABBAkABgAkADQAAwABBAkADgA0AAAAaAB0AHQAcAA6AC8ALwBzAGMAcgBpAHAAdABzAC4AcwBpAGwALgBvAHIAZwAvAE8ARgBMAEMAbwBuAGMAZQByAHQATwBuAGUALQBSAGUAZwB1AGwAYQByAFYAZQByAHMAaQBvAG4AIAAxAC4AMAAwADQAMQAuADAAMAA0ADsAcAB5AHIAcwA7AEMAbwBuAGMAZQByAHQAIABPAG4AZQBSAGUAZwB1AGwAYQByAEMAbwBuAGMAZQByAHQAIABPAG4AZQBDAG8AcAB5AHIAaQBnAGgAdAAgACgAYwApACAAMgAwADEAMQAsACAAMgAwADEAMQAgAEoAbwBoAGEAbgAgAEsAYQBsAGwAYQBzACAAKABqAG8AaABhAG4AawBhAGwAbABhAHMAQABnAG0AYQBpAGwALgBjAG8AbQApACwAIABDAG8AcAB5AHIAaQBnAGgAdAAgACgAYwApACAAMgAwADEAMQAsACAAMgAwADEAMQAgAE0AaQBoAGsAZQBsACAAVgBpAHIAawB1AHMAIAAoAG0AaQBoAGsAZQBsAHYAaQByAGsAdQBzAEAAZwBtAGEAaQBsAC4AYwBvAG0AKQAsACAAdwBpAHQAaAAgAFIAZQBzAGUAcgB2AGUAZAAgAEYAbwBuAHQAIABOAGEAbQBlACAAQwBvAG4AYwBlAHIAdAAuAAIA7gAABCoFeAAXABsARLAcL7AaL7AA3LAcELAL0LALL7AZ3LAAELAd3ACwAEVYsBEvG7ERBz5ZsABFWLAFLxuxBQE+WbARELAY3LAFELAZ3DAxJRQOAiMhIi4CNRE0PgIzITIeAhUFESERBCoSKkY0/jA1RioRESpGNQHQNEYqEv2+AUiALDIbBwcbMiwEeCszGwcHGzMrVvw0A8wAAwB4AAAEEgV4ACYANwBIAHqyKCADK7IFQQMrtNpB6kECXUAbCUEZQSlBOUFJQVlBaUF5QYlBmUGpQblByUENXbIMQQUREjmwBRCwMNywE9ywKBCwONCwBRCwStwAsABFWLAALxuxAAc+WbAARViwGi8bsRoBPlmyOScDK7IMJzkREjmwGhCwKNwwMQEyHgIVFA4EBx4FFRQOAgcGByEiLgI1ETQ+AjMTETM2Nz4DNTQuAicmJwMRMzY3PgM1NC4CJyYnAjJZnXVDGScvLCIHPFI3Hw8DJj5QKWJ7/rg5PhwFBRw+OZiSLyUQHhgODxkgECcxioArIg4cFg0OFx0PJC0FeC5ciVsvTT4uIRMCETU/Qz81EE53Vz0TLgYnQVIsA64rUkAn/Oz+iAIUCBonMyIlNykdCRUEAij+rAISBxghLh4iMyUaCRMEAAABAAoAAAPOBXgAIgAusgwWAysAsABFWLAALxuxAAc+WbAARViwES8bsREBPlmwABCwCtywF9CwGNAwMQEyHgIVFA4CIyMRFA4CIyIuAjURIyIuAjU0PgIzAuooUUIpKUJRKGYFHD45OT4cBWYoUUIpKUJRKAV4Bhw5MzI5HAf8iCtSQCcnQFIrA3gHHDkyMzkcBgAAAQAYAAAGegV4AE4ARACwAEVYsAAvG7EABz5ZsABFWLAPLxuxDwc+WbAARViwQC8bsUAHPlmwAEVYsB8vG7EfAT5ZsABFWLAvLxuxLwE+WTAxATIeAhcTMzYSNzY3NjYzMh4CFRQGBwYHAQYHBgYjIyImJyYnAyMGAgcGBwYGIyMiJicmJwEmJyYmNTQ+AjMyFhcWFxMzPgM3NjYDShwxJxwGrCIqWCwJEhA5LiM1IxECAgIC/u4PGRVDMiIyQBMWC4wiIEkhDRYUPzIiMkUWGg/+8gICAgIRIzUjLjkQEgmuIhMsLS0VDlAFchQjMRz84sMBl8QmHxorGScuFgwWCAoI+8AiGhclJRcaIgJKjv7SjiIaFyUlFxoiBEAICggWDBYuJxkrGh8m/OJcy8/LXTlLAAACAFQAAAOQA+gAKQA+AJSwPy+wCi+wANywPxCwFNCwFC+wKtywD9CwKhCwGtCwChCwH9CwChCwNNCwABCwQNwAsABFWLAFLxuxBQE+WbAARViwDy8bsQ8BPlmyJDkDK7APELAv3EAbBy8XLycvNy9HL1cvZy93L4cvly+nL7cvxy8NXbTWL+YvAl2yCg8vERI5sCQQsBrQsBovsh85JBESOTAxJRQOAiMiLgI1DgMjIi4CNRE0PgIzMh4CFzQ+AjMyHgIVARQeAjMyPgI3NC4CIyIOAhUDkAYePDY3Ph4HCSE0STE3blg3N1huNzFJNCEJHCw3GzI8Hwn9+AQSIx8nMRwLAQscMicfIxIE4ipRQCclNToUHTwwHyRUh2MBImSHUyQeMDweMkInDyc/Uyv+eBs0KhkjSW9NH1FIMhkoMxoAAgBcAAADqAPqADEAPQDHsD4vsDMvsBjcsADQsAAvsD4QsCLQsCIvsAfcsDLQsBgQsD/cALAARViwKi8bsSoFPlmwAEVYsB0vG7EdAT5ZsjMFAyuwHRCwDNxAGwcMFwwnDDcMRwxXDGcMdwyHDJcMpwy3DMcMDV201gzmDAJdsB0QsBPcQBsHExcTJxM3E0cTVxNnE3cThxOXE6cTtxPHEw1dtNYT5hMCXbAqELA43LTZOOk4Al1AGwg4GDgoODg4SDhYOGg4eDiIOJg4qDi4OMg4DV0wMQEUDgIjIRUUHgIzMj4EMzIeAhUUDgIjLgM1NTQ+BDMyHgQVBTM0LgIjIg4CFQOoBBs6Nf5yDB0wIygrGRQhOzQZLyQWUnyRP1OWc0QtSFpcUx5lj2A4HAj95PoFGTUxKS8YBgI6ECsnHGAYNCwcExwgHBMIFicfQVc0FgEoWI5n6l+EWDEYBiQ6TFFTJCYvSTIaGicsEQAAAgBc/nADmgPoAD4AUwDwsFQvsB4vsADcsFQQsCnQsCkvsD/csBbQsBYvsD8QsCTQsD8QsC/QsB4QsDTQsB4QsEnQsAAQsFXcALAARViwBy8bsQcDPlmwAEVYsCQvG7EkAT5Zsi9OAyuwBxCwEdxAGwcRFxEnETcRRxFXEWcRdxGHEZcRpxG3EccRDV201hHmEQJdsAcQsBncQBsHGRcZJxk3GUcZVxlnGXcZhxmXGacZtxnHGQ1dtNYZ5hkCXbAkELBE3EAbB0QXRCdEN0RHRFdEZ0R3RIdEl0SnRLdEx0QNXbTWROZEAl2yHyREERI5sjROLxESObAvELA50DAxBRQOBCMiLgInND4CMzIeAhcWFjMyPgI1NQ4DIyIuAjURND4CMzIeAhc0PgIzMh4CFQEUHgIzMj4CNS4DIyIOAhUDmjFMXVlKE0eOdU0FCBw4MDo+HgkFCC8pJSsWBgkhNEkxN25ZODhZbjcxSTQhCQcePjc2PB4G/fgEEiMfJzIcCwELHDEnHyMSBB5afFIuFwUUNlxIDB4bExMbHw0VESIvMRDgHjwwHiRTiGUBImOHVCQfMDwdFDo1JSdAUSr+dBo0KRkySVEgTHBJIxkqNBsAAAEAegAAA7wFeAA2AGGwNy+wCi+wANywNxCwINCwIC+wFtywK9CyLCAAERI5sAoQsDHQsAAQsDjcALAARViwJi8bsSYHPlmwAEVYsAUvG7EFAT5ZsABFWLAbLxuxGwE+WbIxEAMrsiwQMRESOTAxJRQOAiMiLgI1ETQuAiMiDgIVFRQOAiMiLgI1ETQ+AjMyHgIVET4DMzIeAhUDvAkfPDQ0PSEKBBIjHygyHQsJID41NDwfCQkfPDQ1PiAJCSI2STA3b1c34ipRQCcnQFEqAYoaMygZNV1/Sb4qUUAnJ0BRKgO0KlFAJydAUSr+qh08MB8lVIlkAAIAmgAAAcwFeAAVACEAhLIWHAMrtNoc6hwCXUAbCRwZHCkcORxJHFkcaRx5HIkcmRypHLkcyRwNXbAcELAA0LAAL7AWELAK0LAKLwCwAEVYsB8vG7EfBz5ZsABFWLAQLxuxEAE+WbAfELAZ3LTZGekZAl1AGwgZGBkoGTgZSBlYGWgZeBmIGZgZqBm4GcgZDV0wMRM0PgIzMh4CFREUDgIjIi4CNQEUBiMiJjU0NjMyFpoJIDwzMzwgCQkgPDMzPCAJATJQSEhQUEhNSwMIKlA/Jyc/UCr92CpRPyYmP1EqBABNS0tNUEhIAAEAlv/+A5gFeAAzADWyCgADK7AKELAo0ACwAEVYsAUvG7EFBz5ZsABFWLAhLxuxIQE+WbAARViwLi8bsS4BPlkwMRM0PgIzMh4CFREzNzY2MzIeAhUUBwcTFhYVFA4CIyIuAicnIxUUDgIjIi4CNZYGHDkzNDkbBhDOEjAaGTMnGRy0xBIaHC03GiE0Jx4KcDgGGzk0MzkcBgSaKlA+JiY+UCr+dvIVExMjMR8oItT+iiRJISAyIREgLzYX7qgqUT8mJj9RKgAAAQCMAAABvAV4ABUAKLIKAAMrsAoQsBfcALAARViwBS8bsQUHPlmwAEVYsBAvG7EQAT5ZMDETND4CMzIeAhURFA4CIyIuAjWMCSA8MzQ8HwkJHzw0MzwgCQSaKlA+JiY+UCr8RipRPyYmP1EqAAABAIQAAAPGA+gANQBTsDYvsAovsADcsDYQsCDQsCAvsBbcsCvQsAoQsDDQsAAQsDfcALAARViwBS8bsQUBPlmwAEVYsBsvG7EbAT5ZsjAQAyuwMBCwJtCyKxAwERI5MDElFA4CIyIuAjURNC4CIyIOAhUVFA4CIyIuAjURND4CMzIeAhU+AzMyHgIVA8YKIT00MzwgCQUSIx4pMh0KCiE9NDM8IAkGHj03Nz4fCAgiNUoxN25YN+QrUkAnJ0BSKwGGHDYpGTZef0m+K1JAJydAUisCIitRPyclNDoVHTwwHyRUiGQA) format('truetype');}

]]>

vs referred as an external source:

<style>@font-face {font-family: 'egXPseElB0p1:::Concert One';font-style: normal;font-weight: 400;src: url(https://fonts.gstatic.com/l/font?kit=VEM1Ro9xs5PjtzCu-srDqRTlnveq-hxGBHL1kVp35ZE&skey=1285a0576bf2d826&v=v22) format('truetype');}</style>

B/c an image cannot initiate another network request.

See more here.