HTML that makes use of an API or content delivery service won't work if that service is unavailable. DNS-based service discovery is centralized, and so are most services themselves (at the level of the operating organization, even if the servers themselves are distributed across geography or providers). Downloaded HTML also can't connect to a service that disables CORS[^1], unless you do additional work to serve the HTML from a spoofing server or to a spoofing client. It's typical for API services to disable CORS, and content delivery services to prevent hotlinking.
HTML for a DApp, and IPFS, connect to a local Ethereum node. The local node uses a P2P (decentralized) connection[^3] to the Ethereum network. It therefore relies on the availability of the network, but not of any single org. (Your ISP can still disconnect you, though, at the physical layer or several other layers.)
So first, you cannot save the html of any website like this. A lot of websites generate the content you'r browsing dynamically via a backend. Others will query APIs that will only answer to their domain (CORS).
What I'm saying is that a dapp can be contained into your .html and query the different APIs or the Ethereum network via your own clients/nodes.
