This one’s been kicking about for a while, but I thought I’d document it here.
If for whatever reason you don’t want a Lync meeting to launch in the client, just append “?sl=1” to the end of the invitation URL:
https://meet.contoso.com.au/greiginsydney/ABC123?sl=1
The meeting will launch a page (the image below) that gives you the option of joining from the Lync Web App – a browser window:
Beware that if the presenter’s Conferencing Policy doesn’t permit anonymous participants you won’t get far by trying to join as a Guest: