Short posts on frontend engineering: things I learn, try and break.https://arjunsiv.com/en-gbhttps://arjunsiv.com/writing/a-card-field-that-respects-the-caret/https://arjunsiv.com/writing/a-card-field-that-respects-the-caret/Formatting a card number as you type is easy until someone edits the middle of it. Here’s how to keep the caret where the person put it, plus the Luhn check.Thu, 29 Oct 2026 00:00:00 GMTFormsJavaScriptPaymentshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/a-migration-playbook-for-an-ai-agent/https://arjunsiv.com/writing/a-migration-playbook-for-an-ai-agent/Porting a legacy tool screen by screen is repetitive and easy to get subtly wrong. Writing the rules down as an agent skill made every port consistent.Thu, 29 Oct 2026 00:00:00 GMTAIMigrationProcesshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/an-image-budget-that-fails-the-build/https://arjunsiv.com/writing/an-image-budget-that-fails-the-build/Large images creep into apps one pull request at a time. A small script that runs before every build stops them at the door.Thu, 29 Oct 2026 00:00:00 GMTPerformanceBuild toolsNext.jshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/brand-colours-and-readable-text/https://arjunsiv.com/writing/brand-colours-and-readable-text/When every merchant can pick their own header colour, the text on it has to choose its own colour too. Relative luminance does the work.Thu, 29 Oct 2026 00:00:00 GMTAccessibilityCSSTheminghi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/deadlines-in-days-then-hours/https://arjunsiv.com/writing/deadlines-in-days-then-hours/In dispute management a missed deadline costs real money. Giving the whole product one way of talking about time made it calmer and clearer.Thu, 29 Oct 2026 00:00:00 GMTUXPaymentsCopyhi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/emails-that-list-many-cases/https://arjunsiv.com/writing/emails-that-list-many-cases/Notes from writing fifteen transactional email templates for dispute management, including bulk emails that loop over rows and a naming scheme that kept them sane.Thu, 29 Oct 2026 00:00:00 GMTEmailHTMLTemplateshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/filter-chips-that-dont-fight-you/https://arjunsiv.com/writing/filter-chips-that-dont-fight-you/Two dashboards had each grown their own filter chips, and they had drifted apart. Moving them into a shared design system meant agreeing on a few small rules.Thu, 29 Oct 2026 00:00:00 GMTDesign systemsReactUXhi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/guardrails-for-an-ai-agent/https://arjunsiv.com/writing/guardrails-for-an-ai-agent/I built an in-app design agent for a payments codebase. Most of the work went into what it is not allowed to do, enforced in code rather than in the prompt.Thu, 29 Oct 2026 00:00:00 GMTAISecurityDeveloper toolshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/migrating-without-a-freeze/https://arjunsiv.com/writing/migrating-without-a-freeze/How we moved an operations tool to a new app one screen at a time, with the old and new apps running side by side and talking over postMessage.Thu, 29 Oct 2026 00:00:00 GMTArchitectureMigrationiframeshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/money-inputs/https://arjunsiv.com/writing/money-inputs/An amount field should look friendly to a person and stay exact for the API. Notes on grouping, decimals per currency and limits.Thu, 29 Oct 2026 00:00:00 GMTFormsPaymentsi18nhi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/one-animation-80-kb/https://arjunsiv.com/writing/one-animation-80-kb/How a bundle analysis on a hosted checkout found an animation library that drew a single loader, and what replacing it with CSS did for mobile load times.Thu, 29 Oct 2026 00:00:00 GMTPerformanceWebpackCSShi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/shadows-as-tokens/https://arjunsiv.com/writing/shadows-as-tokens/Our design system had shadows picked by hand in every component. Turning them into named elevation tokens made the whole library feel like one thing.Thu, 29 Oct 2026 00:00:00 GMTDesign systemsCSSTokenshi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/the-build-that-ran-twice/https://arjunsiv.com/writing/the-build-that-ran-twice/Our dashboard took 12 minutes to deploy and 3 to build on a laptop. Reading the pipeline line by line found the app being built twice.Thu, 29 Oct 2026 00:00:00 GMTCI/CDDockerAWShi@arjunsiv.com (Arjun S)https://arjunsiv.com/writing/the-crawler-that-failed-real-payments/https://arjunsiv.com/writing/the-crawler-that-failed-real-payments/Some payments were ending in SYSTEM_ERROR for no reason we could see. The culprit was a text-to-speech feature loading our payment page on its own servers.Thu, 29 Oct 2026 00:00:00 GMTDebuggingPaymentsProductionhi@arjunsiv.com (Arjun S)