Why does my CSS code change when transferring to HTML product link?

[SARSKILD]

Hello, I'm new to coding and wanted to give it a try! I'm adding a button to my product page but when I make test the code on https://www.w3schools.com/ and transfer it to my HTML product link it changes the code. 

Example: 

<a href="https://www.sarskild.com/pages/client-services-book-a-fitting" class="button1" target="_blank">Client Services<br><medium>Enquire A Studio Visit 〉</medium></a>

Changes to 

<a class="button1" href="https://www.sarskild.com/pages/client-services-book-a-fitting" target="_blank">Client Services<br></a><medium>Enquire A Studio Visit 〉</medium> <!-- HTML !-->

Would love to know why! Thank you! 

[Jason]

What is the <medium> element you're adding? Are you hoping to bold the text?

I would suspect the html validator is asking something similar to me and trying to make your code valid.