Web page clutter refers to the situation where too many elements (ads, images, menus,..) or messages are present on a webpage without enough free or white spaces. A cluttered web page is too busy or confusing for visitors.
Web page clutter is an usability issue for the publisher as two many elements or messages on the page compete for visitor’s attention. Then, the visitor can be lost and abandon the visit whithout finding the information or product he was looking for.
As for ad clutter, cluttered web pages are also an issue for agencies and advertisers as it diminishes ad effectiveness (more details on ad clutter).
An example of clutter impact on ad recall (Proximic study):
Web page clutter has to be taken into account by web designers, publishers and advertisers.
An example of "clutter test":