make that two, or consider me as not anecdotal because my opinion is based on documentation. Even if you work out CORS issues, there’s more to consider
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe
… Because each embedded browsing content created by
<iframe>is itself a complete document environment, every use of<iframe>within a page can cause substantial increases in the amount of memory and other computing resources required by the document overall … you should keep the potential for performance issues in mind. …
Accessibility concerns
People navigating with the aid of assistive technology such as a screen reader can use the value of a
titleattribute declared on aniframeto determine what embedded content it contains. … Without this description, they may have to navigate into theiframeand browse around to determine what the embedded content is. This context shifting can be a confusing and time-consuming process …
IMHO as long as there is unifying header navigation, common color palette, common fonts, you don’t need to strive for “identical looking” only “associated looking”.
In fact it could be argued that you are doing visitors a favor by having subtle differences. Just enough so they’ll know “I’m at the same site, but a different part of it, the UI will be this here”.