Simon Willison’s Weblog

Minimal CSS-only blurry image placeholders (via) Absolutely brilliant piece of CSS ingenuity by Lean Rada, who describes a way to implement blurry placeholder images using just CSS, with syntax like this:

<img src="…" style="--lqip:192900">

That 192900 number encodes everything needed to construct the placeholder - it manages to embed a single base color and six brightness components (in a 3x2 grid) in 20 bits, then encodes those as an integer in the roughly 2 million available values between -999,999 and 999,999 - beyond which range Lean found some browsers would start to lose precision.

The implementation for decoding that value becomes a bunch of clever bit-fiddling CSS expressions to expand it into further CSS variables:

[style*="--lqip:"] {
  --lqip-ca: mod(round(down, calc((var(--lqip) + pow(2, 19)) / pow(2, 18))), 4);
  --lqip-cb: mod(round(down, calc((var(--lqip) + pow(2, 19)) / pow(2, 16))), 4);
  /* more like that */
}

Which are expanded to even more variables with code like this:

--lqip-ca-clr: hsl(0 0% calc(var(--lqip-ca) / 3 * 100%));
--lqip-cb-clr: hsl(0 0% calc(var(--lqip-cb) / 3 * 100%));

And finally rendered using a CSS gradient definition that starts like this:

[style*="--lqip:"] {
  background-image:
    radial-gradient(50% 75% at 16.67% 25%, var(--lqip-ca-clr), transparent),
    radial-gradient(50% 75% at 50% 25%, var(--lqip-cb-clr), transparent),
    /* ... */
    linear-gradient(0deg, var(--lqip-base-clr), var(--lqip-base-clr));
}

The article includes several interactive explainers (most of which are also powered by pure CSS) illustrating how it all works.

Their Node.js script for converting images to these magic integers uses Sharp to resize the image to 3x2 and then use the Oklab perceptually uniform color space (new to me, that was created by Björn Ottosson in 2020) to derive the six resulting values.

# 2:44 am / css, css-custom-properties

smartfunc. Vincent D. Warmerdam built this ingenious wrapper around my LLM Python library which lets you build LLM wrapper functions using a decorator and a docstring:

from smartfunc import backend

@backend("gpt-4o")
def generate_summary(text: str):
    """Generate a summary of the following text: {{ text }}"""
    pass

summary = generate_summary(long_text)

It works with LLM plugins so the same pattern should work against Gemini, Claude and hundreds of others, including local models.

It integrates with more recent LLM features too, including async support and schemas, by introspecting the function signature:

class Summary(BaseModel):
    summary: str
    pros: list[str]
    cons: list[str]

@async_backend("gpt-4o-mini")
async def generate_poke_desc(text: str) -> Summary:
    "Describe the following pokemon: {{ text }}"
    pass

pokemon = await generate_poke_desc("pikachu")

Vincent also recorded a 12 minute video walking through the implementation and showing how it uses Pydantic, Python's inspect module and typing.get_type_hints() function.

# 2:57 pm / llm, python, generative-ai, ai, llms, vincent-d-warmerdam

First look at the modern attr(). Chrome 133 (released February 25th 2025) was the first browser to ship support for the advanced CSS attr() function (MDN), which lets attr() be used to compose values using types other than strings.

Ahmad Shadeed explores potential applications of this in detail, trying it out for CSS grid columns, progress bars, background images, animation delays and more.

I like this example that uses the rows="5" attribute on a <textarea> to calculate its max-height - here wrapped in a feature detection block:

@supports (x: attr(x type(*))) {
  textarea {
    min-height: calc(
      attr(rows type(<number>)) * 50px
    );
  }
}

That type(<number>) is the new syntax.

Many of Ahmad's examples can be achieved today across all browsers using a slightly more verbose CSS custom property syntax.

Here are the tracking issues for CSS values support in attr() for Firefox (opened 17 years ago) and WebKit (16 years ago).

# 3:53 pm / css, css-custom-properties, chrome, web-standards, ahmad-shadeed