Rather than using regular expressions, use querySelector
to navigate to the element with the attribute you want to extract, and then use getAttribute
:
const font = document.querySelector('font[color="blue"]');
console.log(font.getAttribute('face'));
<p height="100" width="300">
<span><font color="blue" face="Times New Roman">....some content....</font></span>
</p>
If your input is a string rather than an HTML element, use DOMParser
to turn it into a parsable document first:
const input = `<p height="100" width="300">
<span><font color="blue" face="Times New Roman">....some content....</font></span>
</p>`;
const doc = new DOMParser().parseFromString(input, 'text/html');
const font = doc.querySelector('font[color="blue"]');
console.log(font.getAttribute('face'));