The phrase "a seriousness that" is correct and usable in written English.
It can be used to describe a particular quality or level of seriousness in a situation or context.
Example: "The report conveyed a seriousness that highlighted the urgency of the issue at hand."
Alternatives: "a gravity that" or "a weightiness that".
Of Millar, he writes: "He took his responsibilities seriously, a seriousness that was hidden because he came to be known best of all for his jokes, his style, his elocutio as he put it".
He stared at me with a seriousness that made me uncomfortable.
"There's a seriousness that is out of balance in the N.F.L.," Carroll said.
Some of his work has a seriousness that might horrify him — if indeed he was serious about his frivolous intentions.
He grew serious, but with a seriousness that seemed to be only the stifling of a smile.
She gazed at the sheets of music for a long moment, then started to pound on the keys with a seriousness that took Hanfeng by surprise.
It is, an announcer relates with a mock seriousness that is the show's best feature, "the biggest battle our humble little junkyard has ever seen".
But his BBC work had a heady mix of individuality, creativity and – until the Strauss film – a passionate seriousness that has never been surpassed in arts television.
He possessed an essential seriousness that made him a leader of what some had begun to call "counter-couture".
But Ms. Kowroski also performed with a wholehearted seriousness that made the character recognizably human, as she did last season in the role of the Strip Tease Girl in "Slaughter on 10th Avenue".
Much of the show, however, rests on the shoulders of Billy; and Liam Bower, one of three young actors sharing the role, performs not just with heroic dedication but also a strange seriousness that is affecting.
