This, I think, is a variety of Oxalis. It’s an all too common weed in these parts, but one people do very little to get rid of. It seems to light up the neighborhood when it blooms. Under Oxalis pes-caprae, Wikipedia offers the following common names: Bermuda buttercup, African wood-sorrel, Bermuda sorrel, buttercup oxalis, Cape sorrel, English weed, goat's-foot, sourgrass, soursob, and soursop, along with some less common ones. It’s an ingredient in South African water flower stew. Wikipedia also offers the following under Control:

The plant has a reputation for being very difficult to eliminate once it has spread over an area of land. The weed propagates largely through its underground bulbs and this is one reason why it is so difficult to eradicate, as pulling up the stems leaves the bulbs behind. Soil in which the plant has grown is generally contaminated with many small bulbs.

So it may be that people have just learned to give up and let it bloom. But with poppies and alyssum and buttercup oxalis all blooming at once, there isn’t a single yard that doesn’t look ready for summer. Remember, we’re not growing crops this time of year, we’re growing happiness for as long as it lasts.