Notes

Website Image Tags

Image tags and sizes used for link previews, favicons, and app icons. Platforms change these limits without much notice, and some are not officially documented; these were last checked in November 2026. Values marked with * are from memory or third-party sources, and other values may also rely on secondary sources.

Recommended

This set covers link previews, browser tabs, search results, and home screens on current platforms:

<meta property="og:image" content="https://example.com/preview.png">
<meta property="og:image:width" content="1200">
<meta property="og:image:height" content="630">
<meta property="og:image:alt" content="Description of the image">
<meta name="twitter:card" content="summary_large_image">

<link rel="icon" href="/favicon.ico" sizes="32x32">
<link rel="icon" href="/icon.svg" type="image/svg+xml">
<link rel="apple-touch-icon" href="/apple-touch-icon.png">
<link rel="manifest" href="/manifest.webmanifest">

The icons section of manifest.webmanifest is below; web app install also needs name or short_name, start_url, and display:

{
  "icons": [
    {"src": "/icon-192.png", "type": "image/png", "sizes": "192x192"},
    {"src": "/icon-512.png", "type": "image/png", "sizes": "512x512"},
    {"src": "/icon-maskable-512.png", "type": "image/png", "sizes": "512x512", "purpose": "maskable"}
  ]
}

The 1200x630 image suits pages with a designed preview image, and fills the width of a message in chat apps. If the only image is a logo or icon, use a 280x280 square image instead so it appears as a small thumbnail next to the title, set the og:image:width and og:image:height tags to match, and use twitter:card summary. Do not reuse the 512x512 app icon for previews, since WhatsApp will show it at full size.

• Use absolute https:// URLs for preview images
• Use PNG or JPEG for preview images, and keep important content centered in case of cropping
• Keep preview images under 600 KB, which is WhatsApp’s documented limit
• Serve favicon.ico from the site root, since some tools only look there
• Give the Apple touch icon an opaque background and square corners
• Keep the logo in a maskable icon inside a centered circle with a radius of 40% of the icon width
• sizes="32x32" reportedly stops Chrome from preferring the ICO over the SVG
• Change the filename when replacing an image, since platforms and browsers cache them aggressively

Exhaustive

Link Previews

X falls back to og:image when twitter:image is missing, but needs twitter:card to choose the card format.* X accepts JPG, PNG, WEBP, and GIF, and only uses the first frame of an animated GIF.*

Chat apps choose between a large preview and a small thumbnail using either the image’s dimensions or the page’s tags:

WhatsApp documents the requirements for a large preview, and says it may relax them or revert to small previews without giving a threshold, so a thumbnail under 300 pixels wide comes from the same report. Signal does not use a single width threshold, so a large square image may still appear as a thumbnail. Discord shows a small image beside the text by default, accepts PNG, GIF, JPEG, WEBP, and AVIF, and fetches the image to measure it unless og:image:width and og:image:height are set.

Preview Image Properties

Structured Data

Favicons

App Icons

Legacy

These are safe to skip on new sites:

Sources:

• The Open Graph Protocol
• Meta; Images in Link Shares
• LinkedIn; Make Your Website Shareable
• X; Summary Card with Large Image
• Discord; Link Previews
• Google; Favicon in Search Results
• Google; Article Structured Data
• Evil Martians; How to Favicon
• Can I Use; SVG Favicons
• Apple; Configuring Web Applications
• web.dev; Maskable Icons

Timezone Boundaries Mapped

The IANA timezone database contains over 400 timezone entries mostly named as “<continent>/<city>”. They are meticulously cataloged to group regions of the world not only by UTC offset but also by the presence and timing of Daylight Savings as well as current and historical timezone policy changes.

The github.com/albertyw/localtimezone repository maps latitude/longitude data to IANA timezone names. It is based on github.com/evansiroky/timezone-boundary-builder which in term is based on OpenStreetMap. From that data, I generated an Interactive Map:

Comparing WebdriverIO Firefox vs Chrome

Chrome recently started releasing arm64 linux builds which meant that my ubuntu-on-m1-mac can run webdriverio (wdio) builds on both firefox and chrome (it was previously stuck doing only firefox builds).

I therefore compared build speed of codemancer on firefox versus chrome.

wdio.conf.ts Configuration

Firefox:

capabilities: [{
  maxInstances: 5,
  browserName: 'firefox',
  'moz:firefoxOptions': {
    args: ['-headless'],
  },
  acceptInsecureCerts: true,
}],

Chrome:

capabilities: [{
  maxInstances: 5,
  browserName: 'chrome',
  'goog:chromeOptions': {
    args: ['--headless=new', '--no-sandbox', '--disable-dev-shm-usage', '--disable-gpu'],
  },
  acceptInsecureCerts: true,
}],

For Arm architectures, the most recent version of wdio still depends on @puppeteer/browsers at ^2.2.0 which doesn’t correctly fetch Chrome’s Arm builds. Therefore, these additional configs need to be added to wdio.conf.ts:

// Set these environment variables to manually-downloaded chrome executables
// For the satantime/puppeteer-node:26-trixie-slim docker images, use
// CHROME_BINARY=/usr/bin/chromium
// CHROMEDRIVER_BINARY=/usr/bin/chromedriver
const chromeBinary = process.env.CHROME_BINARY;
const chromedriverBinary = process.env.CHROMEDRIVER_BINARY;

// inside capabilities
'goog:chromeOptions': {
  args: [/* ... */],
  ...(chromeBinary ? { binary: chromeBinary } : {}),
},
...(chromedriverBinary ? { 'wdio:chromedriverOptions': { binary: chromedriverBinary } } : {}),

Benchmarks

I benchmarked both browsers on codemancer unit tests over 30 iterations.

On Arm64, Chromium 151 against Firefox 155:

browser    runs      min   median     mean      max
chrome       10   4934ms   6593ms   7143ms   10054ms
firefox      10   7055ms  11426ms  17360ms   50600ms

On x86_64, Chromium 152 against Firefox ESR 140:

browser    runs      min   median     mean      max
chrome       10   4433ms   5265ms   6798ms   15222ms
firefox      10   6645ms   9119ms  18613ms   56509ms

References

• WebdriverIO browser runner
• Chrome for Testing availability
• Bringing Chrome to ARM64 Linux Devices
• puppeteer#14258: Installing Chrome with --platform linux_arm installs x86-64 binary

Geolocation Precision

Latitude/longitude and other geolocation systems can express a position at exponentially increasing precision as you add digits or levels. Below are tables of the approximate ground precision of several coordinate systems.

One degree of latitude is roughly constant everywhere (~111 km), but one degree of longitude shrinks toward the poles as the meridians converge, scaling with the cosine of the latitude. For the latitude/longitude systems below, the east/west distance is therefore given at several latitudes. Grid systems like H3, S2, Geohash, and Plus Codes are designed to be nearly uniform across the globe, so their precision is instead given as an approximate cell edge length and area.

Degrees with decimals (Degrees)

Degrees-minutes-seconds (DMS)

Geohash (base32)

Each geohash character adds 5 bits, alternating between longitude and latitude, so cells are square at odd lengths and 2:1 (wider than tall) at even lengths.

Plus Codes (Open Location Code)

H3 hexagonal geospatial indexing

S2

UTM (Universal Transverse Mercator)

UTM projects each of 60 longitudinal zones (each 6° wide) onto a flat grid and expresses a position as an easting/northing in meters within the zone. Because the coordinate is already metric, the precision is simply how many digits of the easting/northing you keep.

MGRS (Military Grid Reference System)

MGRS is a grid-based alphanumeric encoding built on UTM (and UPS at the poles): a grid-zone designator, a 100 km square identifier, then equal numbers of easting and northing digits. Each added digit pair narrows the cell tenfold.

Maidenhead locator system

Used by amateur-radio operators, Maidenhead alternates letter and number pairs (field → square → subsquare → …) to encode longitude and latitude. Cells are 2:1, wider than tall, because longitude spans 360° against latitude’s 180°.

Interpreting Linux Memory Metrics

note

Linux memory accounting is full of metrics that look similar but measure very different things. This post is a short tour of the most useful ones and where they come from.

How Linux Models Memory

Linux gives every process a private virtual address space. Pages in that space may or may not be backed by physical RAM at any given moment - the kernel pages them in on demand and evicts them under pressure. Many pages are also shared between processes (libraries, the page cache, copy-on-write forks), so “how much memory does this process use?” does not have a single right answer. It depends on whether you count what is reserved, what is resident, what is private, and how shared pages are split across processes.

On top of that, the kernel itself uses memory for the page cache, slab allocators, buffers, and reclaimable structures. “Free” memory is rarely the number you actually care about, because the kernel will happily use almost all RAM for cache and return it on demand.

Per-Process Metrics

From /proc/<pid>:

• VSZ (Virtual Size): Total size of the process’s virtual address space, including code, data, mapped files, and unallocated reservations. Pages that are mapped but never touched still count. Useful as an upper bound, not as a usage number.
• RSS (Resident Set Size): Amount of physical RAM currently backing the process’s pages. Shared pages are counted in full for every process that maps them, so summing RSS across processes overcounts system memory.
• PSS (Proportional Set Size): Like RSS, but each shared page is divided by the number of processes sharing it. Summing PSS across all processes approximates total physical memory in use.
• USS (Unique Set Size): Private resident memory only - the portion that would actually be freed if the process exited. The truest measure of a process’s incremental cost.
• WSS (Working Set Size): The set of pages a process actively touches over a given time window. This is the minimum RAM the process needs to run without thrashing. Not a static counter; tools like wss.pl sample idle/young page flags to estimate it.
• Swap: Pages that have been pushed out to a swap device. Visible per-process in /proc/<pid>/status (VmSwap) and smaps.

System-Wide Metrics

From /proc/meminfo:

• MemTotal / MemFree: Total RAM and the small slice that is completely unused. MemFree is almost always misleadingly low on a healthy system.
• MemAvailable: Kernel’s estimate of how much memory is available for new allocations without swapping, accounting for reclaimable cache and slab. This is the field to watch.
• Buffers / Cached: Page cache used for block devices and file contents. Reclaimable on demand.
• SwapTotal / SwapFree: Swap device capacity and remaining space.
• Dirty / Writeback: Pages modified in cache that still need to be flushed to disk. Spikes here can stall writes.

Pressure and Reclaim

Raw “used vs free” numbers do not tell you whether the system is struggling. Two further signals do:

• PSI (Pressure Stall Information): Exposed at /proc/pressure/memory and per-cgroup at memory.pressure since kernel 4.20. Reports the percentage of wall time over 10s/60s/300s windows during which some tasks (some) or all tasks (full) were stalled waiting on memory reclaim. Even small non-zero values indicate the system is doing real work to find memory.
• pgscan / pgsteal / pgmajfault in /proc/vmstat: Counters for how aggressively the kernel is scanning the LRU lists, reclaiming pages, and servicing major faults from disk. Rising rates here usually precede visible slowdowns.
• pswpin / pswpout in /proc/vmstat: Pages swapped in from and out to disk. Non-zero pswpin means the system is paying real disk I/O to bring previously-evicted pages back into RAM - a direct indicator that the working set no longer fits.

When to Use Which

• Capacity planning for a single process: USS for incremental cost, WSS for the RAM it actually needs to run without thrashing.
• Adding up processes to explain total RAM: PSS. RSS double-counts shared libraries; USS undercounts them.
• Sizing a container or cgroup memory limit: WSS plus headroom for the page cache the workload depends on. Setting the limit at RSS is a common cause of surprise OOM kills.
• “Is the box about to OOM?": MemAvailable trending down plus PSI memory.full above zero. MemFree alone will mislead you.
• Investigating a slowdown that “has plenty of free memory": PSI, pgscan, pgmajfault, and Dirty/Writeback. High scan rates with low fault rates mean the kernel is working to keep up; high major-fault rates mean it is losing.
• Diagnosing a memory leak in a long-running process: watch RSS and USS over time. Rising USS is a real leak; rising RSS with flat USS is usually shared-library or cache growth.
• Comparing two processes that load the same big library: USS, since RSS will charge each one the full library cost.
• Deciding whether to add swap: SwapFree trending toward zero combined with non-trivial PSI memory.some means the system is already paging under pressure.
• Tuning a database or file server: Cached and Buffers should be large - that is the page cache doing its job. Worry when Dirty spikes or Writeback stays elevated.
• Choosing what to alert on in production: MemAvailable, PSI memory.full avg60, and swap-in rate (pswpin in vmstat). These three together catch nearly every memory-related incident before users do.

Further Reading

• PSI - Pressure Stall Information (kernel.org)
• proc_meminfo(5) man page
• PSI: pressure stall information for CPU, memory, and IO (LWN)
• Brendan Gregg - Working Set Size Estimation
• Understanding Memory Metrics: RSS, VSZ, USS, PSS, and Working Sets (Blackwell Systems)
• Process Memory Management in Linux (Baeldung)
• cgroup v2 and Page Cache (Viacheslav Biriukov)
• Getting Started with PSI (Facebook)

Coordinate to Timezone Lookup Library

Go Compression Benchmark Results

California Oak Tree Identification

Claude Code With Ollama Setup

Switching From Python requirements.txt to pyproject.toml

Generating Cloudflare Origin Certificate for Multiple Domains

Introductory Computer Science and Software Engineering Topics

PID Controller

Availability Percentages

Javascript/Typescript Decorators Suck

Upgrading MariaDB Database Versions

Concurrent Python Example

Updating UUIDField on MariaDB to Django 5

Replacing Setup.py

Fixing Mariadb --Column-Statistics Errors

Geographic Geometry Simplification

Linters

Installing Mysqlclient in Python Slim Docker Image

Processor Trends

Resizing a Ubuntu Disk in a UTM VM

Bash File Test Operators

Python Generic Type Annotations

Mac Menubar Applications

Logodust

Python Releases

Debian Releases

Ubuntu Releases and Support Periods

Monitoring System CLIs (Top for X)

Fixing "EFI stub: Exiting boot services and installing virtual address map..."

ARM Support

Map Caps Lock to Escape for Vim

Download and Convert Youtube Playlists to MP3 Files

Nobody Ever Got Fired for Copying FAANG

Removing Token Authentication From Jupyter/iPython Notebooks

Debian and Ubuntu Releases

Setting Up FastAI Fastbook on a Fresh Ubuntu Instance

Tip for Developer Tools Startups

A Better Go Defer

Covid-19 Economy Predictions

Basic Docker Monitoring

Switching From Go Dep to Go Mod

Upgrading LibMySQLClient in Python MySQLDB/MySQLClient

Developing Django in Production

Quote

Sendmail Wrapper for Mailgun

Python Release Support Timeline

Use the Default Flake8 Ignores

Making Pip Require a Virtualenv

Engineering Toolbox

Node Timezones

Sampling Samples

Rotating a NxN Matrix in One Line of Python

iTerm2 Search History

Nginx Auth With IP Whitelists

Bash Strict Mode

Optimizing Asus Routers for Serving Websites With Cloudflare

Browserify, Mochify, Nyc, Envify, and Dotenv

Scraping Images From Tumblr

There Are Too Many NPM Packages

Programmers Writing Legal Documents

Solidity Review

Likwid

My First Server's IP

Installing Netdata

Interrobang Versus Shebang

Bad Interview Questions

Showing Users in Different Databases

Some MIT (Undergraduate) Admissions Interview Advice

Optimize the Develop-Test-Debug Cycle

Example of Python Subprocess

Spotted in Taiwan

Fixing "Fatal Error: Python.h: No Such File or Directory"

Cassandra Primary Keys

MyPy Review

Griping About Time Zones

Bundling Python Packages With PyInstaller and Requests

Go Receiver Pointers vs. Values

Fixing statsonice.com Latency

Showing Schemas in Different Databases

Straight Lines

Emerson on Intellect

Core Metric for Developer Productivity

How to Capture a Camera Image With Python

Python Has a Ridiculous Number of Inotify Implementations

Projects: Gentle-Alerts

Creating a New PyPI Release

Eva Air USB Ports

Projects: Git-Browse

Cassandra Compaction Strategies

Code Is Like Tissue Paper

Seen in a Bathroom Stall at MIT

Underused Python Package: Webbrowser

Pax ?

Golang Review

Wadler's Law

Tunnel V2

MultiPens

SSH Tunnel

That Time I Was a Whitehat Hacker

Comparison of Country and Company GDPs

Sketching Science

Tech Hiring Misperceptions at Different Companies

Calculating Rails Database Connections

DevOps Reactions

Tuning Postgres

Fibonaccoli