The name Writely reminds me of a side project I worked on around 2010. I was not satisfied with the performance of Google Docs and its competitors at the time like EditGrid and thought (naively, as it turned out) I'd be able to develop a faster alternative.

I had no idea what I was doing and thinking that using JavaScript to manipulate the DOM was going to be slow, I chose ActionScript and Flash as the language and runtime to develop the project in. I wrote a client-side expression parser and formula engine, and managed to develop a functioning spreadsheet UI with resizable rows and columns, copy and paste with Excel-like animations, cell references etc.

The problem that I ran into was text-rendering when there was a lot of text on the screen. The application would consume a lot of memory and the page would slow down to a crawl when scrolling. I couldn't really find a way to speed up the performance and stopped working on the application after some time. That's when I realized the incredible amount of work that went into Google Docs and other web-based spreadsheets. :)