He is best known for being the cofounder and CTO of Slack (software), as well as co-owning and developing the online creative community B3ta with Denise Wilton and Rob Manuel; being the chief software architect for the photo-sharing application Flickr (originally working for Ludicorp and then Yahoo) and writing the book Building Scalable Web Sites for O'Reilly Media. He has also worked for EMAP and is responsible for writing City Creator among many other websites, services and desktop applications. Cal was the co-founder and VP of engineering at Tiny Speck, the company whose internal tool transitioned into Slack (software).
He is color blind, and has worked on applications to make the web more accessible to the color blind. He is also a frequent contributor to open source software projects and runs a number of utility websites, such as Unicodey, to make certain programming tasks easier.