Some browsers lack support for iframes due to their old nature, while others lack it because they were built expecting to receive information in a single http request. Specific phone browsers also adapt to the limited computing power of the phone by not supporting iframes.
Iframes are used to embed content from other external sources, such as YouTube videos, and almost all current browsers support them. Other browsers that lack the support for this feature include text browsers that focus on the text in a webpage and ignore the rest. These don’t necessarily need the iframes, as they are mainly used to embed graphic content. Particularly old browsers, such as previous versions of Opera before Opera 5, Netscape 4 and any browsers using the Gecko core, are not equipped to handle the use of iframes.