Officially supports Magento 2.2.0+

Download it

composer require nathanjosiah/magento2-layout-debugger:dev-master

Register it with Magento

bin/magento setup:upgrade

For production mode also run

bin/magento setup:di:compile
bin/magento setup:static-content:deploy

Go to Store->Configuration->Advanced->Developer->Debug

Outputs a popup containing information about the rendered layout for a given page. The popup can be expanded and hidden.

Will add unobtrusive comments as to not interfere with CSS selectors and rendered HTML that help you quickly see where HTML is coming from in the layout.

This will be the name of the layout element, the type of the element (block, container, ui component), and the name of the parent element