When you are working locally on an already running application, and if you have changed pages/css etc. You can run your application again to get a hot deployment which will quickly update the application with the changes you’ve made and refresh your page in your browser.
So you don’t have to wait on a full deployment. And you can view the application in your browser as it’s viewed by as the user instead of the View mode.
Is that something you are looking for?
for styling in SASS we use this Gulp file
https://medium.com/@jasonteunissen/how-do-i-start-styling-in-mendix-gulp-sass-6b37ddaf8de6
It compiles your sass and updates the page realtime.
As for mendix, i think the page just updates when you hit run locally