I want to make a PowerPoint add-in to show some small html+css component on each slide, and not by adding add-in every time I'm adding a new slide. What should I look for and could it be done with some office add-ins and commands APIs?
In PowerPoint, the way to add content that's repeated on every slide is called Slide Master (View > Slide Master). Unfortunately, content add-ins in the Slide Master view show only the static image of the add-in, not live HTML+CSS. So there's no way to put live HTML+CSS on every slide with a single operation, but if all the user needs is the image of the content, it will work.
Note that the user can actually insert copies of the same content add-in quickly to multiple slides with copy+paste, and keyboard shortcuts can make it quick.
Another alternative is to consider whether a task pane add-in could achieve your goal. If the content doesn't need to be visible during slide shows, you'll be able to see it on the task pane for every slide.
来源：https://stackoverflow.com/questions/36063158/overlay-item-for-all-slides-in-powerpoint