^^Data URL.  MIME type.   base 64 code.

data:[<mediatype>][;base64],<data>   sintassi Data URL

mediatype   is a MIME type string

base64        token to say that the data are non-textual, but base64 coded.

                   ref: wp/Base64.

                   ref: rapidtables/image-to-base64  encoder online

MIME type    Multipurpose Internet Mail Extensions

a standard that indicates the nature and format of a document, file, or assortment of bytes.

Es di MIME type

type/subtype   is the syntax

text/plain

text/html

image/jpeg    for a JPEG image file

type/subtype;parameter=value is the syntax with parameter

text/plain;charset=UTF-8

Encoding bytes to printable char

1. wp/Base64.
2. johndcook/base 85 encoding | base 32 and  base 64 encoding
3. rapidtables/image-to-base64  encoder online

Links

1. wp/MIME
2. developer.mozilla/HTTP/Data_URIs
3. developer.mozilla/HTTP/MIME_types