to add on to the above comments, below is a frond-end designer and developer workflow suggested by Adobe in one of the sessions.
Regarding the front end stack you want to do, the technology can vary based on how you want to proceed.
If you are going to have your frontend stack outside AEM, then you can build as per the current industry standard - NodeJS, ES, SASS, gulp, webpack etc. and use plugin to create aem package of these application. If you want to write frontend code in AEM itself, you can focus on HTML5, LESS, JS
Now a days, SPAs have so much relevance, so if you want to start with it, react is the best supported SPA framework for AEM.