Validation

Links

http://validator.w3.org/ W3C Markup Validation Service.
CSE HTML Validator. The Lite version of this product immediately found the error I was looking for.
HTML Tidy Library Project

W3C Markup Validation Service.

Use the online service https://validator.w3.org/

Or download the jar file from https://github.com/validator/validator/releases/

I downloaded:

cd ~/repo/temp/
wget https://github.com/validator/validator/releases/download/15.6.29/vnu.jar_15.6.29.zip
unzip vnu.jar_15.6.29.zip
cp ./dist/vnu.jar ~/bin/

Run the validator:

java -jar ~/bin/vnu.jar http://localhost:8000/links/

HTML Tidy

sudo apt-get install tidy

I downloaded an HTML page from our site using HTTPie:

http GET http://localhost:8000/links/ > out.html

Run tidy:

tidy --indent auto --indent-spaces 2 source.html > out.html 2> tidy.out