I didn't realize that built-in a fallback.  Is it a JavaScript-powered fallback?  Have you considered just doing a css fallback for browsers that don't support svg:

```
#header .logo {
  background-image: url(../img/logo.svg);
  ...
}

.lte9 #header .logo {
  background-image: url(../img/logo.png);
  ...
}
```

That's why the conditional IE classes are added to the `<html>` tag, for simply css-driven conditional replacement.