The International Digital Publishing Forum (IDPF) created the open source ePUB file type. Based on XHTML, XML, and CSS, it integrates nicely with a variety of software programs, which has increased the format's popularity. However, there are a number of requirements that must be met when creating an ePUB file, and this has led to criticism of the format. The format enables the addition of DRM security to a file.
Reflowable content is possible with the ePUB format. In the same way that the text on an HTML page alters according to the size of the display, this means that the content fits the size and form of the screen it is being displayed on. Three components typically make up an ePUB file: the archive, the metadata, and the content.