When you're on the site, hit ctrl + shift + i to load the developer tools. In the developer tools, click the kebab menu (3 vertical dots) near the top-right of the developer tools. In the Menu, goto "more tools" and then "Network conditions". The network condition settings will appear at the bottom. For the network condition settings, check "disable cache". Then, uncheck "select automatically" for the user agent. Then, in the user-agent drop-down, set Chrome for Windows. Then, hit ctrl + r to reload the webpage.
If things load correctly when you do that, it means the site is serving Opera broken code/HTML.