A browser that does not recognize the <script> tag at all, will display the <script> tag's content as text on the page. To prevent the browser from doing this, you should hide the script in comment tags. An old browser (that does not recognize the <script> tag) will ignore the comment and it will not write the tag's content on the page, while a new browser will understand that the script should be executed, even if it is surrounded by comment tags.

In addition to hiding the script inside a comment, you can also add a <noscript> tag.

The <noscript> tag is used to define an alternate text if a script is NOT executed. This tag is used for browsers that recognize the <script> tag, but do not support the script inside, so these browsers will display the text inside the <noscript> tag instead. However, if a browser supports the script inside the <script> tag it will ignore the <noscript> tag.