When he asks her to marry him, it is at precisely 9:48 PM, Western Standard Time. There are no blimps or fireworks or the UCLA marching band playing in the background. He doesn't propose in front of a crowd or even at a secluded restaurant. He doesn't spell out her name in chocolates or flash the question across a stadium screen. He of course considered all these things and more (including but not limited to constructing an animatronic puppy that will bark out the question or inscribing his proposal across the roof of his Malibu home with his repulsors and flying her overhead) but he knows Pepper isn't in it for the glamour.
Nonetheless, he does take the weekend of his proposal off (silences his cell, blocks all Stark Industries bulletins, and hides his Avengers Alert card underneath a stack of sound-muffling pillows) to do all the expected, uncomprehendingly exorbitant things that come in a Tony Stark relationship. Okay, so this may be his first actual relationship, but still, being associated with him does imply a level of grandeur in celebrations. They continent-hop between the U.S. and Western Europe, landing in Paris for her favorite designers, chartering the Stark Alpha yacht on a waterside tour of the Spanish coastline, and finish off in Venice because they never made it there when he originally suggested it. They drink expensive white wine and the finest of liquors his fortune can provide. After the Europe expedition they dock in New York long enough to walk around Central Park all incognito-like (only recognized once, a success for two celebrities) and pick-up her favorite espresso and coffee beans that are impossible to ship to the West Coast. They jet home and land in a field near his mansion so they can avoid airports and the unappealing paparazzi awaiting them.
Pepper remains blissfully ignorant the whole time since this is Tony Stark and he is bound to random bouts of eccentricity like cross-Atlantic jaunts.
They settle back into their home (because it may be his property, but it'll always be their home) and while Pepper showers and redresses he prepares her favorite meal for them (an innate sense of chemistry and molecular relationships are useful for more than just the construction of a superhero suit). Barefoot and hair curling at the ends, she emerges in a corset-style sundress the same color as the backless gown she wore the first time they danced. She's never looked more beautiful, but then she smiles and it's even better.
They pop in her favorite movie, an Audrey Hepburn classic. It doesn't really matter which one because Pepper loves the actress in anything. The lights dim and invisible blinds cover the glass walls to block out stray sunlight. They eat and laugh and add commentary which anybody else would find annoying but they find endearing and together they generally ignore the other seven billion people in the world.
He's steadfastly watching the clock on the movie screen the entire time, which resides in the upper corner directly adjacent to the weather (a warm 74° at the time, thanks J). He can feel the velveteen box in his pants pocket, stashed there since he lifted it out of the hidden compartment in his workshop. The sensation is a constant reminder of his imminent proposal and an inkling of nervousness trickles into his person. But he's Tony Stark and his confidence alone could sell someone a majority share in a Fortune 500 Company, so he plows over the nerves like a bulldozer and refocuses on the present.
Pepper is lying in his arms, secure and warm. They banter over the audio of the movie while he plays with her hair and she nuzzles his arc reactor which glows in the darkness of the room. She's engaged enough in the plotline as to ward off any possibility of snoozing (something he definitely considered, since waking a girl to propose to her is a little awkward) and he stays there in their little slice of perfection.
At 9:40, JARVIS's accented voice fills the room with his and Tony's secret code for the proposal notification; the announcement piques Pepper's curiosity and she glances at him with questioning eyes to which he informs her that it's merely a stray experiment update and no cause for concern. Tony knew the alert would be unnecessary in all scenarios but he wanted every base covered to eliminate any possibility of a mishap. Still, Tony takes this as his cue and shifts Pepper so he's free to move around. She looks a little annoyed at the loss of touch, her lips doing that little quirky thing that he loves so much, but she makes no move to reinitiate contact. He pauses the movie and light again fills the room as the window shades rescind and the fluorescent lights brighten.
She is unsuspecting at first thanks to his convincing cover story, but soon grows suspicious as he makes no move to leave and attend to the "experiment" in the workshop. Instead, he lowers himself to the ground and starts talking. Her quick-thinking mind realizes what's going on within 1.2 nanoseconds, the weekend leading up to this and a few uncharacteristic choices before that sliding into place now, and a brilliant smile appears on her face.
He doesn't speak much, since he's never been one for words except for press releases and business conferences where she's usually the one telling him what to say anyway. He sums up their relationship beautifully and eloquently, mixing in the five shared languages they know and a few chunks of technobabble that only Pepper could put into context.
He finishes, just shy of 9:48, with "… and I am yours, 112%."
He punctuates this by pulling out the previously hidden box and flipping open the lid to reveal the most exquisite ring either of them have ever laid eyes on.
"Will you be the Iron Wife to my Iron Husband?"
Someone on the outside might see this as cheapening the moment - but for them it's the perfect amount of humor, said with the perfect undertone, and the most perfectly intense gaze in his dark chocolate eyes.
She responds with a nod, furiously bobbing her head to communicate her desires, then hurls herself at him as he slips the custom-tailored ring onto her finger. This ring is metallic perfection and melds to her skin as though it was always meant to be. There will be no resizing because he is the best metal-worker in the world and this is his greatest creation.
As the metal meets her skin every technology-based surface of the house displays a blinking 9:48 in huge, emboldened font. Confusion flashes across her face before she lovingly asks him the significance of the glowing numbers.
He obliges, breaking down the time to individual digits. The nine is for the number of years she's put up with his antics and not surprisingly the best nine of his life. Four is the number of years he's been Iron Man and she has always been (and will always be) his motivation. He hesitates slightly before divulging his reasons behind his last choice; anyone else would have missed the brief pause, but she's Pepper so she catches it and silently encourages him to continue with a brush of her soft fingers over his jawline. The eight, he says, isn't really an eight. It's infinity, because that's how long they're going to stay together and that's how long he'll love her.
She's already in tears before he describes his ring selection.
It is not gaudy, oversized, or tacky. A diamond cut to match the outline of his arc is bookended by two shimmering rubies the same glistening red of his suit. But the metal is the true magic. It's not sterling silver or plain platinum. He tells her how it was crafted out of the same element he discovered, the one that saved his life and currently acts as the core to his arc reactor. This is the only ring in existence made of that metal, the only other item in the world outside of his body that is forged from that substance.
Finally, he slips the ring off her finger and flips it around so that Pepper can read the inscription that curls around its interior. It's an elegant calligraphy that is so paper-thin it is practically non-existent and one would have to know to look for it to find it. Tony tells her this inscription is done in the titanium-gold alloy that comprises his suit and it does glint gold when it catches the light.
Once she can see past the twinkle, she reads it. Written in gorgeous Italian, it reads: you're the proof