I'd like to view emails only in plain text when using Gmail in the web browser. Is this possible (and how do I make it the default)?
Unfortunately, you can't. There have been themes in the past (notably "Terminal") which used a fixed-width font, but even then the messages were still rich text/HTML.
If you want to see a message in its plain-text glory, all you can do is "show original" from the message menu. That'll give you the raw email including headers.
You can compose/send messages as plain text, but that's not what you're asking.
You can also use the "Message text garbled?" option in the dropdown menu triggered by the triangle next to the Reply button. Like "Show Original", this will also clutter it with assorted junk, but usually not nearly so much as with full headers.
Unfortunately, it also is not "sticky", i.e., is not a permanent answer to your problem. If you come across one, please post here, as I would love to find an answer! (Especially as LinkedIn has made their emailed discussion comments nearly useless in "rich" text....)