Today I installed the After The Deadline plugin to test it out.
I use the HTML editor by default. I opened a draft post, and clicked the [PROOFREAD] quicktag button. AtD scanned the draft and returned no errors. However, the plugin apparently changed all of my DOCTYPE-compliant lower-case HTML tags to non-compliant ALL-CAPS tags.
I can find no way to modify this behavior. The plugin offered no option to ignore whatever "rule" modified my HTML tags, and there are no configurable options on either the AtD plugin options page or on my profile page.
Why on earth would a modern proofreading plugin change HTML tags to a format that has not been standards-compliant for several years now?