As I guess you know, iOS limits websites based on the options you've selected in Settings > General > Restrictions > Websites.
As @grgarside shows with the screenshot in their answer, with Restrictions you can either:
- Allow all websites
- Limit Adult Content
- Allow specific websites only
By using these settings you can also set whitelists etc.
Based on your question, I assume you've limited adult content and every so often you'll try accessing a site that iOS has restricted.
With the Limit Adult Content filter enabled, there are several layers of filtering applied by iOS to web browsing. Apple has achieved this via a number of methods:
- Applying an automatic filter layer to prevent direct access to many adult sites
- Enabling the various search engine based filtering options to prevent inappropriate terms from being queried independently
- Adopting the likes of Google SafeSearch and similar from Bing, Yahoo, and others
- The prevention of web searches for a wide variety of terms.
In your question you offered a single example via the Yanflex screenshot. However, since Yanflex is only a 3rd party method of accessing Cragslist, the likelihood in this specific case is that iOS has restricted something from the Craigslist servers. This is not surprising, as specific individual pages on Craigslist regularly appear in block lists because of their content.
The only way around this is when you encounter a specific page that is blocked that you don't want blocked, you will need to specifically allow it by going to Settings > General > Restrictions > Websites > Limit Adult Content and adding it to the Always Allow list. That is assuming you have enabled Limit Adult Content to begin with.