You're using Visual Studio for this? Wouldn't be my first choice for web development - I am a big fan of Notepad++:
Notepad++ Home. It's very fast, lightweight and can also deal with pretty large files (the logs I analyse for Windows Update can be 250,000+ lines long and N++ can handle them).
Anyway, my code works for me in Pale Moon... What is the contents of Idea.html btw?
Are you just loading this page from a folder on your computer? From what I've read, localstorage has issues when not running on a web server. A good local server setup is XAMPP:
https://www.apachefriends.org/index.html