In the end, it's all about planning and collaboration between the frontend and the backend. Taking time to plan new features and discuss the data transfer objects that will travel between the front- and backend will solve most of the theoretical issues.
If you would decide to build the frontend first, you can always use a tool like Swagger to have a temporary dummy API the frontend can communicate with.
I was just about to write a response with this exact sentiment. For me, it's all about defining a contract between the two early, then building to fulfil that contract.
Once the contract is in place, I don't see a significant advantage or disadvantage to starting on either side of the fence.
My go-to tool for creating dummy APIs is json-server. It's much less feature-rich than Swagger, but also much more lightweight.
If you wanna go ever more lightweight, you can paste your JSON on Gist and then just link to that (get the URL via the "Raw" button on top right).
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