Web Workers is a specification that defines an API which allows Web application developers to use background workers running scripts in parallel. Web Workers is used with JavaScript and is platform-independent. Hence, Web applications written in JavaScript can be used for a wide variety of purposes. There are many Web applications and some of them, for instance Internet election campaign and real-time broadcasting, need secure communications. We can include digital signatures with such Web applications to guarantee their security of communications. In this demonstration proposal, we assess the performance of parallel implementations of public key cryptosystems on severalWeb browsers (Internet Explorer, Google Chrome, Opera, and Firefox). With our implementation, it is possible to verify a message in 1.03 milliseconds on a Windows PC and 5.49 milliseconds on an Android tablet (Nexus 7). We also propose cryptography applications for Web browsers.