But if you're serious about reducing carbon waste, don't make visitors download 7.5MB to load the webpage.

Have you seen vert.sh? It does on-device conversion of audio, images, and documents and is open source too.

And if you happen to have wasm or web workers disabled, you get an unhelpful "Processing 0/0 files…" message that just hangs forever.

Of course, if you meant "use the png format" instead of "look at the header", that's likewise unhelpful.

Not sure if there's actually a suggestion in here. I entered the site thinking "if you want a jpg, this is the app". Any question I had about other formats was moot because this is the app for jpgs. But then you discussed the other formats so it seemed like you had some workarounds or ways to use jpgs in place of the other formats, and that left me unsatisfied whereas your saying nothing probably wouldn't have? Again, not sure if there's anything actionable in there, just letting you know my experience with the site.

Where I could make one HTTP request and get a json blob and the associated image at the same time.

Then separate them on the client side and add them to the Dom.

I've run some WebP images from reddit through your site and they all end up looking terrible. If I run them through Squoosh.app (MozJPEG, 75%), I get the expected minimal degradation in quality and a 30%ish reduction in size. If I run the same image through your site (which seems hard-coded to Q90 for quality), I get a minimal reduction in size (if any; to be expected given Q90) and it looks incredibly pixelated.

If I give it a jpeg, it works great: a large reduction in size and I can't even notice a difference (which is kinda what I expect for Q90).

EDIT: If I toggle on the "legacy mode" YCbCr (instead of XYB), the WebP files work great. If I get the jpeg from reddit instead of the webp, XYB is fine.

Try an image like https://www.reddit.com/r/PeterExplainsTheJoke/comments/1o8wo... (you can right click and download the image to get the webp or "download linked file" to get the jpeg). If I put the jpeg into your site with XYB, there's very slight degradation and a 67% savings. If I put the webp in, it looks like it was resized to half or a quarter of the size and then enlarged back up (and it gained 57%). With webp and YCbCr, it gained 63% over the webp, but the image quality is good.

So there's something with the WebP images and XYB that seems like it isn't working.

EDIT 2: I just grabbed an AVIF I have around to test and the XYB seems to have the same problem there as well.

This is a personal project, and the engagement here is both exciting and incredibly valuable.

I've already pushed a fix for the WebP/AVIF quality issue based on the excellent report from mdasen—thank you!

I wanted to address a few other major themes you all brought up:

1) "JPG isn't for everything" (especially screenshots/art):

You are 100% right.

The goal of OnlyJPG isn't to argue that JPG is the best format for every use case (it definitely isn't!).

The philosophy is: "When you are in a situation where you need a JPG for compatibility, you should be able to make the best possible one."

I've seen firsthand how often people need to convert screenshots or diagrams to JPG for a CMS, an email client, or a social media site that mangles PNGs. The new default of 4:4:4 subsampling actually helps a lot with text and sharp lines, but I'll be looking into adding a dedicated "Graphics" mode that's even better tuned for this.

2) UX & Copy (especially the transparency section):

The feedback that the copy was confusing is also spot on (thanks catapart!). It's hard to see those things when you're close to the project. My goal was to explain why you might choose JPG over other formats, but it came across as a confusing sales pitch. I'm going to rewrite those sections to be clearer and more direct about what the app actually does (e.g., "How OnlyJPG handles transparency from a PNG").

3) Performance, Trust, and Open Source:

I have issues with the large initial page load, the question of trust, and the fact that the project isn't open-source. You're right on all counts.

Performance: The ~7.5MB load is too high. Optimizing the WASM bundles and vendor libraries is now at the top of my to-do list.

Trust: The best way to verify the "100% in-browser" claim is, as craftkiller noted, to load the page and then disconnect your internet. If it still works, you know nothing is being uploaded.

Open Source: This is the big one. To be transparent, I'm a 40-year-old self-taught developer, and this is one of my first real apps. I was never introduced to GitHub and haven't gotten into the rhythm of open-sourcing my work. But the passionate feedback here has convinced me it's the right next step. I commit to open-sourcing OnlyJPG on GitHub. Your expertise and willingness to help have shown me the immense value of building in public.

This has been an incredible (and slightly overwhelming!) learning experience. Thanks again for taking the time to test, critique, and advise.