Before coming here, I reported this as a bug to SurveyMonkey. I received the following reply.

Thanks for reaching out to notify of these the issues that you've noted within Opera.

At this time, Opera is not one of our supported browsers, which is likely why you're encountering these issues. To ensure optimal performance, we recommend utilizing a version of one of our supported browsers.

Please refer to this article for more info about our supported browsers: https://help.surveymonkey.com/articles/en_US/kb/What-browser-versions-do-you-support

Steps to reproduce: Visit www.surveymonkey.com in Opera.
Actual result: A plain text web site with no styles.
Expected result: A well-formatted web site, like I see in Chrome.
OS: 10.11.6
URL: www.surveymonkey.com
Version of Opera: 47.0

Errors such as the following appear in the developer console on www.surveymonkey.com. They may hint at the cause of the issue.

[Report Only] Refused to load the stylesheet 'about:blank' because it violates the following Content Security Policy directive: "default-src https: data: 'unsafe-eval' 'unsafe-inline' 'self'". Note that 'style-src' was not explicitly set, so 'default-src' is used as a fallback.
Redirect from 'https://csp.surveymonkey.com/report?a=prod&e=false' has been blocked by CORS policy: Redirect location 'about:blank' has a disallowed scheme for cross-origin requests.