Less 5 alpha.1: Jess-powered compiler preview

[matthew-dean]

Summary

This PR opens the Less 5 alpha.1 preview on top of the current upstream alpha branch. Less 5 is the first Less release powered by the Jess stylesheet engine, a shared CSS-family compiler engine for modern stylesheet languages. The aim for this alpha is to let people try the new compiler path early while the remaining Less 4 compatibility surface is still being filled in.

Less 5 alpha.1 feature checklist

New in the Less 5 direction

• Native CSS nesting is supported and preserved by default, matching the platform direction now tracked on caniuse; the nesting fixture shows nested output preserved as authored.
• collapseNesting: true and lessc --collapse-nesting are available when flattened selector output is desired. This mode does not try to join separate @media / @supports wrappers the way Less 4.x sometimes did: separate at-rules cannot be joined generally, and attempting that can recreate the historical combinatorial explosion that extend/nesting used to trigger. CSS at-rule nesting for @media / @supports has been supported since 2013, while native CSS selector nesting has been supported since 2023.
• Less-style ampersand suffix joining still works, including BEM-style selectors like .block { &__item { ... } }, dash suffixes like &-item, modifier forms like &--primary, and numeric suffixes like &1.
• Collapsed nesting preserves cascade order, so declarations written after nested children stay after those children in emitted CSS.
• Generated nested/extended selectors may use modern CSS selector forms such as :is() where that is the cleanest representation.
• The extend engine is substantially improved: extends work through nested selector structures and collapse into readable modern :is() forms where appropriate. The updated extend fixture shows the input case, and the generated output shows the cleaner nested-extend result.
• Modern CSS-heavy syntax is part of the parser direction, including nested rules and current at-rule surfaces such as layers and container queries.
• Less now builds on Jess's spec-compliant CSS parser, so the alpha can catch invalid CSS that Less 4.x often accepted or passed through.

Ready for alpha testing

• The less package installs as a thin package that uses the Jess stylesheet engine directly.
• less.render(), less.renderFile(), and lessc are wired for the supported alpha surface.
• Core Less workflows are covered: variables, arithmetic, mixin calls, sibling file imports, and nested-rule output.
• CLI and Node API smoke tests pass for the alpha package.
• The alpha fixture gate runs across the Less test-data corpus with explicit expected gaps.
• Friendly Jess diagnostics are surfaced through the Less wrapper for supported parser/eval failures.
• Release automation is alpha-aware and covered by release-flow tests.
• Packed-consumer verification passes against the published Jess alpha runtime.

Not ready yet

• Legacy plugin host APIs: @plugin, render-option function plugins, file-manager plugins, and pre/post-processors are not alpha.1 execution paths.
• Source maps are not an alpha.1 publish gate.
• URL rewrite/process-import compatibility, including urlArgs and remote/import URL behavior, is still pending.
• Compression/minification parity is not targeted by this alpha.
• Browser/Sauce harness coverage returns later.
• Some permissive Less 4 parser corners intentionally reject or remain tracked as explicit fixture gaps.

Verification

• pnpm run test:release
• pnpm run test:alpha
• Fork CI on matthew-dean:alpha passed

This is an alpha preview, not the final Less 5 compatibility line. The checklist above is intended to make the current contract easy to review and easy to extend in follow-up PRs.

[greptile-apps]

Too many files changed for review. (423 files found, 100 file limit)

Bypass the limit by tagging @greptile-apps to review.

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[puckowski]

I will have some time this Friday (7/31) and this weekend so I will review soon. @matthew-dean

[matthew-dean]

@puckowski Thanks! Note this is of course the first v5 alpha. There's probably other holes / bugs and other rough spots I missed.

Also, I of course do NOT expect a review of 423 file changes. It's a complete replacement of the AST, parsing, and eval engine entirely. I think the first test is just some real world lessc compile testing. And this would be a merge into the alpha branch would publish under the NPM alpha tag.

[matthew-dean]

@puckowski I'll try to leave Discord open if you have questions -- https://discord.gg/rb5EXYmpB

[matthew-dean]

Merging (alpha only) so this is actually easily testable (under an NPM alpha tag)