This program just compiled Ok for me:
const char webpage[] PROGMEM = R"=====(
<html>
<head>
<script src=\"https://cdnjs.cloudflare.com/ajax/libs/svg.js/2.6.6/svg.min.js\"></script>
</head>
<body>
</body>
</html>
)=====";
void setup() {
}
void loop() {
}
'//' within a quoted string did not cause a problem. With your example, I did get an error message that the variable 'webpage' had to be const to put it in Flash memory so I added the 'const' keyword, but that is the only change I made.
If you do that, and your compiler is still complaining, there is some other issue than the '//'.