There are several solutions to this. Using Textastic is one. It can handle sftp and it has an extensive extra keyboard row with more characters than you will ever need.
Coda also has sftp support and is a very good HTML editor.
My current solution is to use a combination of FE File Explorer Pro and Kodex. It does not have an extra keyboard row with special characters like /, < and >, if that is important to you. I use an external keyboard so special characters is not an issue for me. It does, however, have other interesting possibilities.
Kodex is a code editor that use the native iOS Files interface to open files. It has many good features that I like. One is the scroll bar to the right. It lets you quickly scroll through the open document.
Kodex doesn’t have ftp or any other file transfer protocol. That’s where File Explorer comes in. File explorer can connect using several ftp protocols as well as most of the common cloud services (Box, Dropbox, Onedrive, Google Drive and many more). What makes it special is that once you have created a connection in File Explorer, you can add that as a connection in the iOS Files app. That way Kodex can open remote html files like if they were local files.
In the image you see screenshots from Textastic, Coda and Kodex.