Arduino IDE c++ 11 raw string literal have issues with // in http://, complier thinks // is comment
Escape the two quote marks inside the string using a backslash (literal quotes).

Without the escape characters, the URL is "outside" of a string because the quote marks are string delimiters.

char webpage[] PROGMEM = R"=====( 
&lt;html&gt;
  &lt;head&gt;
    &lt;script src=\"https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js\"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;/body&gt;
&lt;/html&gt;
)=====";