Documentation at https://melpa.org/#/impatient-mode

See the effect of your HTML as you type it.

impatient-mode is a minor mode that publishes the live buffer through the local simple-httpd server under /imp/live/<buffer-name>/. To unpublish a buffer, toggle impatient-mode off. Start the simple-httpd server (`httpd-start') and visit /imp/ on the local server. There will be a listing of all the buffers that currently have impatient-mode enabled. This is likely to be found here: http://localhost:8080/imp/ Except for html-mode buffers, buffers will be prettied up with htmlize before being sent to clients. This can be toggled at any time with `imp-toggle-htmlize'. Because html-mode buffers are sent raw, you can use impatient-mode see your edits to an HTML document live! This is perhaps the primary motivation of this mode. To receive updates the browser issues a long poll on the client waiting for the buffer to change -- server push. The response happens in an `after-change-functions hook. Buffers that do not run these hooks will not be displayed live to clients.