Use anchor tag for normal links and underline on hover.

Use anchor tag for normal links in chat for more native experience
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Using anchor tags has the advantage that the browser displays it as a normal link, like all links on the internet. It lets you right click the link, copy the URL to clipboard, open in tab or window, or incognito window, etc. It's better to use the HTML standard than try to mimic such behavior with custom JavaScript when possible.

Note that this is only used for regular links that are posted in chat; the internal WebIRC javascript-based links (like joining a channel, etc.) remain as they are.

Underline links on hover
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This is a purely aesthetic change. I think it looks and feels nicer, and but I'd need to use it for longer to ensure it feels good.