With weddings this year being delayed, downsized or split into two — video ceremony now, party next year — many guests have some only-in-2020 etiquette questions. Should you dress up? Send a present — or two? Read on for answers to all your pandemic wedding quandaries.

Zoom to the chapel.

Laura Held, founder of Ida Rose Events in Arlington, Va., said couples are now hiring videographers to livestream the events instead of just having a laptop open to Zoom. “They are saying, ‘if I’m going to broadcast it, I want it done well,’” she said.

This means guests might receive a link where they can watch, but — unlike in Zoom — the couple can only see how many people are online, not who they are (or what they’re wearing). If you want to be a great guest, you could get dressed up, take a selfie and post it to social media, saying that you’re getting ready for the wedding.

“It’s flattering for the bride and groom — it shows you’re excited to be a part of their special day,” Ms. Held said. (You can post screen shots of the ceremony to something fleeting, like Instagram Stories, she said, but it’s best to avoid doing so on your main feed until the couple has had a chance to share.)

Can I wear my sweatpants?

If you’re attending a wedding on any platform where you’re in front of a camera, dress in a way that you’re happy to be photographed. Hasti Maloufi, a wedding planner in Vancouver, British Columbia, now incorporates into the wedding rehearsal how the couple will acknowledge grandparents who couldn’t travel or guests in other time zones. (Ms. Maloufi now sends the wedding party down the aisle more slowly, “so the camera has time to pan in and pan out, and get the next person walking.”) You can just turn off your camera, but beware that everyone will assume it’s because you’re not dressed.

It may not be enough to just wear a nice shirt. Virtual weddings may feature group dances and karaoke so you may also want to wear pants or a skirt, unless you’re deliberately going for laughs.

Caroline Creidenberg, founder of Wedfuly, a Denver-based online wedding planning company, said she has seen guests ham it up during their toasts by wearing — in one case — traditional Filipino formal wear on top and what looked like sweatpants printed with a giant wolf on the bottom. Others have paired sport coats with bathing suits. Ms. Creidenberg, whose company has put on some 500 weddings since the pandemic started, pointed out that if a couple has specifically chosen a videoconferencing option they want online guests to be involved. Whatever you wear, “these couples have been through the wringer, so the best thing you can do is be attentive,” she said.

Do I mute my mic?

Bethel Nathan, an officiant in San Diego who has performed about 40 weddings during the pandemic, said most couples seem to be muting everyone during the ceremony. If the couple doesn’t mute you first, it’s wise to mute yourself to prevent interruptions from people or pets at home.

Ms. Nathan has been in business for 12 years, but in recent months she’s had to adapt her ceremony to give a warning that she is about to pronounce the couple as married. “You have to kind of turn to the camera and say, ‘Are you all ready?’ and then someone has to unmute everyone if the couple wants to hear the cheers,” she said.

Help! I was asked to be there — IRL.

If you’re attending a wedding in person, you may be asked to quarantine and to take a coronavirus test. Greg Moss, 36, and Danielle Black, 36, who married on the Brooklyn Grange rooftop farm on Sept. 26, asked their 20 guests to self-isolate for two weeks before the wedding and submit proof of a negative coronavirus test by email.

“We understand this is a lot to ask,” the couple wrote on their wedding website, which also included a list of rapid testing sites. “We’re relying on you, our most special people, to honor us and each other with your honesty and strict caution.”

A couple of guests did not attend because they could not comply — “with good reason,” Mr. Moss said in an interview, “but everyone understood where we were coming from.”

How will the reception be different?

One thing guests — both in person and virtual — can expect is shorter receptions. (Ceremonies often seem to be similar length as in the Before Times, with the main difference being that more couples are writing their own vows, according to several wedding planners interviewed.)

Leah Weinberg, a wedding planner who founded Color Pop Events in Queens, said a “normal” wedding would be about six hours from guest arrival to the end of the night. Currently, the events are about three hours, she said, because in-person parties of 20 or 30 people don’t need an hour for cocktails, and there isn’t usually much dancing.

Virtual guests don’t necessarily want to watch the in-person ones eat dinner, so some receptions with a large virtual guest list may go directly from the couple being pronounced married to the first dance, any family dances (such as father daughter), toasts and cake-cutting. Then the virtual guests log off, and dinner is served (yes, after the cake), Ms. Creidenberg said.

Another feature of some virtual weddings: Breakout rooms, which the couple organizes, à la tables at a wedding. “Keep your breakout rooms to the first page of gallery view of Zoom,” Ms. Creidenberg said — in other words, to fewer than 25 people per room. If you’d prefer not to chat with your “table,” you can always turn off your camera and mute yourself until the couple arrives, said Brittany Ward, lead planner at Modern Rebel, a wedding planning company based in Brooklyn.

“You’re expected to stay until the couple shows up to say hello, but it’s a virtual event, so logging off is not uncommon if someone can’t stay and wait,” Ms. Ward said. One tip for guests: Switch to gallery view so you can see everyone in the room and not just the person speaking at the moment.

When do I give a gift?

First, keep in mind that you are never obligated to give a gift, said Diane Gottsman, founder of the Protocol School of Texas, in San Antonio. If there’s a video wedding to which you’re invited, followed by a larger in-person celebration planned for next year, give the gift soon after the video wedding.

“That’s the wedding,” she said. “No one knows what’s going to happen next year. Plan only for what you’re invited to at that moment.”

If you do go to the celebration next year, you can choose to give another gift or just bring along a bottle of wine, depending on your relationship with the couple and your finances.

If you were invited to an in-person wedding, then uninvited because of Covid-related downsizing, ask yourself how you would feel the next time you see the couple if you haven’t acknowledged the wedding in some way. “If you’re going to dread it, then there’s your answer,” Ms. Gottsman said. You don’t need to buy them a gift; you could just send a really beautiful card congratulating them, or maybe tuck in a funny old picture you found, she said. Anything that shows a little bit of effort.

But wait. I wasn’t invited to the wedding.

Every expert interviewed had the same advice about being left out: Don’t take it personally. It wasn’t because you did something wrong. The best gift you can give the couple is to be compassionate and give them space.

You could also do one better. When Ram Johal, 28, and Margaret Hermano, 29, had to downsize their Vancouver wedding — even significant others of the wedding party didn’t make the cut — a group of about 20 of their friends had an idea. The group asked the wedding planner, Ms. Maloufi, if they could watch the ceremony on their phones (with Zoom) from the church parking lot, then surprise the couple with in-person, socially distanced congratulations. Ms. Maloufi brought a portable speaker so the couple could dance their first dance, to “You Are the Reason,” by Leona Lewis and Calum Scott, as their friends cheered.

But note to anyone thinking of wedding crashing: Venues have strict rules about numbers, so run your scheme by the planner. “Someone should be in the loop to ensure it’s seamless and memorable — for the right reasons,” Ms. Maloufi said.