This approach has two issues. First: if the main site restructures their DOM or CSS, it can break the external site. Second: content changes do not automatically get propagated to the third-party sites. This can lead to inefficiencies and extra costs in maintaining and supporting those sites. It can also cause a lack of brand control and an inconsistent cross-channel customer experience, which can erode site conversion and brand loyalty.
This, of course, requires coordination and integration with the external third party, which is not always a forgone conclusion – it may not work for everyone. But in the right environment, this approach has the power to greatly reduce the level of effort to maintain consistent customer experiences and can improve conversion rates and revenue – something we can all rally around.