I'd be glad to hear from anyone who has more information on this. So far as I know, there is no definitive answer to this question. The legislation predates the web and I don't know of any court cases that have decided the matter.
Many councils are being cautious for the moment and publishing both on the web and on noticeboards. This both definitely conforms with the legislation and also avoids accusations of excluding people who lack internet access. Although central government is more and more insisting that everything be done on the web.
Being pragmatic, I would take into account whether the meeting is on a regular schedule and whether there is any matter of significance to be decided. If there is an important decision to be made, I'd be inclined to rearrange the meeting, or defer that particular matter. If there are only minor matters, it is probably reasonable to go ahead.
If decisions are made unlawfully, they could be challenged in court. But, from a practical perspective, you can ask yourself whether anyone would think it worth embarking on expensive litigation in order to have a council decision declared unlawful. For minor matters, it is very unlikely that anyone would.
A less costly approach would be a complaint to the auditor. Again, the question of the significance of the decisions arises, as for minor matters the worst that would be likely to happen would probably be a comment by the auditor on the annual return telling the council to do better next year.
The possibilities raised above are worst case scenarios. You'll see that I'm hedging my bets!
