Unfortunately they should fire the project manager for the iWeb and OS X app updates as they tend to remove lots of features. Photos is basically the old Finder now. If you have your photos already organized in a folder, finder is it better and faster than photos. (if you don't plan to edit in photos and tend to use 3rd party photo editors).
Also since photos can be open while iWeb is open you need only use keyboard commands or menu system to copy the image form photos to clipboard then paste it to iWeb. Yes you loose the drag and drop ability.
The biggest shame is that OS X Terminal App has the best window tweaks built into it in its Preference and Profiles section. Which can be includef in any OS X app. This give the biggest ability to make your apps transparent.
I would rather have this feature in EL Capt. rather 2 windows side by side. (Are the serious?)
iWeb is still the Best WYSIWYG web editor which support PHP, HTML, HTML 5, Java photo albums and more.
iWEB was support video backgrounds long before they became as popular.
Also since now most browsers and mobile browser handle frames better, nesting frames is not as big as an issue if you know how to set your targets correctly.
The biggest issue is that IE still is so ancient, the reflection features doesn't work. So becareful when using advance image effects in iWEB.
However you can emulate most any website include flash style sites with iWeb.