Android devices are currently known for their poor HTML signature support in the majority of email apps.
Currently none of the official Gmail, Outlook, Yahoo apps can let you install HTML signatures. However, here are three work arounds that may work for you:
Use Gmail or Outlook web apps.
You can use Gmail and Outlook in your browser by going to gmail.com or outlook.com respectively. You will be able to insert HTML coded signature in email settings in both of the apps.
Use Samsung Mail (for Samsung Galaxy s8+ and newer).
If you are holding Samsung Galaxy s8+, s9 or any newer version that came out, you can simply use your default Samsung Mail app that would let you install HTML signature in Email Signature Settings of your app by copying it and pasting it there.
Use third-party apps and sync your email with it.
There are a few apps on the Market that can handle HTML signatures. Currently, we have discovered: Nine, Aquamail and Newton.