pandoc/test/command/7099.md

```
% pandoc -f html -t native --verbose
<iframe src=""></iframe>
^D
2> [INFO] Skipped '<iframe src></iframe>' at input line 1 column 16
[]
```

```
% pandoc -f html -t native --verbose
<iframe src="h:invalid@url"></iframe>
^D
2> [INFO] Fetching h:invalid@url...
2> [WARNING] Could not fetch resource h:invalid@url: Could not fetch h:invalid@url
2>   InvalidUrlException "h:invalid@url" "Invalid scheme"
2> [INFO] Skipped '<iframe src="h:invalid@url"></iframe>' at input line 1 column 29
[]
```