It seems to be using a somewhat similar concept it calls "incidental white space", in order to limit the embedding of leading whitespace (unlike Python which just stores the entire triple-quoted string as-is), but is more permissive:

Text content can be indented less than the trailing delimiter, which I understand is an error in Swift

    String html = """
    ..............<html>
    ..............    <body>
    ..............        <p>Hello, world</p>
    ..............    </body>
    ..............</html>
    ..............    """;

The trailing delimiter can also be on the same line as the final content, which suppresses the final (new)line:

    """
    line 1
    line 2
    line 3"""

translates to

    "line 1\nline 2\nline 3"

whereas

    """
    line 1
    line 2
    line 3
    """

translates to

    "line 1\nline 2\nline 3\n"

I believe this requires escaping the newline in Swift instead (nb: according to the JEP this would also work in Java).