xslint

A linter for XSL/XSLT stylesheets. It first checks that every stylesheet is well-formed and every XPath expression compiles, then flags stylistic, semantic, and logical mistakes — each pinpointed to its exact line and column, with a fix where the correction is unambiguous. Think of it as the checks ESLint gives JavaScript, but for XSL.

Run it

npx @maxonfjvipon/xslint path/to/stylesheets

It also runs in your editor — VS Code and compatible editors and JetBrains IDEs — in CI via the GitHub Action, or embedded through the language server.

Manual

What it reports

[ERROR]   sheet.xsl(2:1) The xsl:output instruction is missing. (not-using-output)
[WARNING] sheet.xsl(3:3) The match attribute starts with //, which scans the whole tree. (starts-with-double-slash)
[WARNING] sheet.xsl(4:5) A variable has a single-character name. (short-names)

Proven on real code

Pointed at core stylesheets from the three most widely-used XSLT projects — DocBook-XSL (1.0), TEI (2.0), and DITA-OT (1.0/2.0), 70 files in all — xslint surfaced 1,974 findings across 22 different checks, with no false positives from its validators: 106 xsl:choose blocks with no xsl:otherwise, 67 unused named templates, 40 stylesheet functions never called, and more. Real stylistic and logical findings in code that has shipped for decades.

Checks

71 checks, each with its rationale. A run reports the 33 marked recommended unless asked for --preset all: what a processor refuses, and the dead code whose report is almost never wrong.

Check	Type	Severity	Description
blank-nested-if	xpath	warning	An xsl:if holds nothing but another xsl:if, stating one condition in two places. Combine the tests with 'and' in one xsl:if, unless the outer test guards the inner one.
duplicate-param-name	xpath	error recommended	Two xsl:param siblings share a @name. Each parameter of a template, function, or stylesheet must have a distinct name.
duplicate-with-param-name	xpath	error recommended	A single call passes two xsl:with-param with the same @name. Each parameter can be supplied only once.
empty-choose	xpath	error recommended	An xsl:choose has no xsl:when branch. Add one, or drop the xsl:choose and keep its content directly.
empty-content-in-instructions	xpath	warning recommended	An instruction element such as xsl:for-each or xsl:if has no content. Add content or remove the empty element.
empty-variable	xpath	warning	An empty xsl:variable binds the empty string without saying so. State the value with select="''" or give it some content.
function-complexity	xpath	warning	The function contains more than 50 XSLT elements. Split it into smaller functions.
function-template-is-not-child-of-stylesheet	xpath	error recommended	An xsl:function or xsl:template is nested inside another element. Move it up to the top level, as a child of the stylesheet element.
function-use-in-xslt-1	xpath	error recommended	An xsl:function requires XSLT 2.0 or later, and the version in force here is below that. Use a named template, or raise that version to 2.0.
incorrect-use-of-boolean-constants	xpath	warning suggested fix recommended	The test is the string 'true' or 'false', a non-empty string that is always true, not a boolean. Use true() or false() instead.
malformed-version-in-stylesheet	xpath	error recommended	A declared version is not a decimal number, so it names no XSLT version and every check gated on one stays quiet. Declare it as a decimal, such as 2.0.
missing-id-in-stylesheet	xpath	warning	The stylesheet element has no @id attribute. Declare it to specify the unique identifier explicitly.
missing-or-empty-href	xpath	error recommended	The @href is missing or empty, so this pulls in no module and no processor will load the stylesheet. Name the module to pull in.
missing-or-empty-name	xpath	error recommended	The @name is missing or empty, so it holds no QName and no processor will load this stylesheet. Give it a name.
missing-version-in-stylesheet	xpath	warning suggested fix	The version attribute is missing on the stylesheet's root element. Declare it to specify the stylesheet version explicitly.
mode-or-priority-without-match	xpath	error suggested fix recommended	An xsl:template that has no match attribute must have no mode attribute and no priority attribute. Add match attribute or remove mode or priority.
modern-construct-in-xslt-1	xpath	error recommended	This XSLT 2.0 construct stands where the version in force is below 2.0, which a conformant 1.0 processor rejects. Use a 1.0 equivalent, or raise that version to 2.0.
name-starts-with-numeric	xpath	error recommended	A variable, parameter, function, or template name starts with a digit, so no processor loads this stylesheet. Rename it to start with a letter.
not-creating-attribute-correctly	xpath	warning	xsl:attribute with a static name on a literal element can be a literal attribute (an AVT for a computed value). Write it inline.
not-using-attribute-test-in-if-when-node	xpath	error recommended	An xsl:if or xsl:when has no @test attribute. Add one, or remove the element.
otherwise-not-last	xpath	error recommended	An xsl:otherwise must be the last child of its xsl:choose. Move it after every xsl:when.
oversized-template	xpath	warning	The template contains more than 100 XSLT elements. Decompose it with apply-templates into focused templates.
param-after-content	xpath	error recommended	An xsl:param must be declared before any other content in its xsl:template or xsl:function. Move it above the content.
setting-value-of-variable-incorrectly	xpath	warning	A variable or parameter is assigned via a nested xsl:value-of instead of the select attribute. Use select syntax instead.
short-names	xpath	warning	A variable, parameter, function, or template has a single-character name. Use a descriptive name that reveals intent.
sort-not-first	xpath	error recommended	An xsl:sort must come before any other content in its xsl:for-each or xsl:apply-templates. Move it to the front.
stylesheet-has-no-templates	xpath	warning	The stylesheet declares nothing at all, so it contributes nothing to a transformation. Add a declaration to it.
template-has-no-name-or-match	xpath	error recommended	An xsl:template must have a @match or @name attribute, or both. Add one of them.
text-outside-xsl-text	xpath	warning suggested fix	Literal text appears directly inside an XSLT instruction element. Wrap it in xsl:text.
too-many-templates	xpath	warning	The stylesheet declares 10 or more templates, too many to read in one file. Split it into imported modules.
use-choose-without-otherwise	xpath	warning	xsl:choose has no xsl:otherwise branch. Add xsl:otherwise to handle unmatched cases explicitly.
use-single-option-for-choose	xpath	warning	xsl:choose contains only one xsl:when branch. Replace it with xsl:if.
using-disable-output-escaping	xpath	warning suggested fix	disable-output-escaping='yes' is set, which produces implementation-defined output. Use xsl:copy-of or literal elements instead.
using-not-outermost-stylesheet	xpath	error recommended	An xsl:stylesheet, xsl:transform or xsl:package is nested inside another element. It must be the outermost element of the document.
variable-or-param-with-select-and-content	xpath	error suggested fix recommended	An xsl:variable, xsl:param or xsl:with-param has both a select attribute and a non-empty body. Use only one way to set the value.
when-or-otherwise-outside-choose	xpath	error recommended	An xsl:when or xsl:otherwise is valid only as a child of xsl:choose. Move it into an xsl:choose, or use xsl:if.
with-param-use-in-invalid-parent-node	xpath	error recommended	xsl:with-param stands under an element taking no parameters at the version in force, which a processor of that version refuses. Move it into a call, or raise the version.
with-param-without-name	xpath	error recommended	xsl:with-param requires a @name attribute to bind the parameter it passes. Add one.
unreachable-function	corpus	warning recommended	A stylesheet function is called only from functions that are themselves never reached, so it can never run. Remove it, or call it from a template.
unused-function	corpus	warning recommended	A stylesheet function is never called in any expression across the corpus. Remove it or call it.
unused-named-template	corpus	warning	A named template is never invoked via xsl:call-template from anything that runs. Remove it or call it.
unused-variable	corpus	warning	A variable is declared but never referenced by $name across the corpus. Remove it or use it.
invalid-xpath-expression	validation	error recommended	An XPath expression is malformed and cannot be parsed. Fix its syntax.
malformed-stylesheet	validation	error recommended	The stylesheet is not well-formed XML and cannot be parsed. Fix its syntax.
syntax-newer-than-xslt-version	validation	error recommended	This XPath syntax belongs to a later XSLT version than the one in force, so a conformant processor rejects it. Raise the version, or respell the construct.
broken-href	format	error recommended	The @href names no file on disk, so no processor will load this stylesheet. Point it at the module, or restore the file.
circular-import	format	error recommended	The stylesheet pulls in itself, or a module that pulls it back, through xsl:import or xsl:include, which is a static error. Break the cycle.
confusing-variable-and-node	format	warning suggested fix	A variable name is used bare as a node selector. Use $name syntax to reference the variable.
count-compared-to-zero	format	warning safe fix	count(...) is compared with 0 to test existence, which walks the whole sequence. Test existence directly instead (exists()/empty() on XSLT 2.0+, boolean()/not() on 1.0).
leaking-result-namespace	format	warning suggested fix	The xsl:stylesheet declares a namespace prefix used only in stylesheet logic, which a literal result element copies into the output. Add the prefix to exclude-result-prefixes.
name-compared-to-string	format	warning suggested fix	name() or local-name() is compared with a string, which is prefix-fragile and slower than a node test. Match the element with a node test on the self axis instead.
not-creating-element-correctly	format	warning	xsl:element is used with a static name. Use a literal result element instead.
not-using-output	format	warning	The xsl:output instruction is missing. Declare it to specify the serialization format explicitly.
output-method-xml	format	warning suggested fix	xsl:output declares method='xml' while the root template builds an HTML document. Change the method to 'html'.
predicate-position-literal	format	warning safe fix	A positional predicate is written the long way; [position() = N] is just [N] and [position() = last()] is just [last()]. Shorten it.
redundant-boolean-call	format	warning safe fix	A boolean() call stands where nothing but a truth is taken, so the wrapper is redundant. Drop it.
redundant-double-negation	format	warning safe fix	A double negation not(not(x)) is redundant; it is just boolean(x), and where nothing but a truth is taken simply x. Replace it.
redundant-import	format	warning suggested fix recommended	The stylesheet imports or includes the same module more than once. Keep a single xsl:import/xsl:include of it and remove the other.
redundant-namespace-declarations	format	warning suggested fix	The xsl:stylesheet declares a namespace prefix that nothing in it uses. Remove the declaration, or exclude the prefix where a literal result element copies it out.
redundant-whitespace	format	warning safe fix	An XPath expression has redundant whitespace. Remove the extra spaces.
scans-whole-document	format	warning	An expression opens a path with //, which scans the whole document each time it runs. Name the path, use xsl:key, or bind the scan once in a top-level variable.
starts-with-double-slash	format	warning safe fix suggested fix	A pattern alternative opens with //, which at most demands a document-node root. Drop it, and set an explicit priority if the template must rank as before.
string-length-compared-to-zero	format	warning suggested fix	string-length(...) is compared with 0 to test emptiness, which measures the whole string. Compare the value with '' instead.
template-writes-nothing	format	warning recommended	This template declares variables and writes nothing to the result tree, so applying it produces nothing. Write the output it was meant to build, or empty it.
translate-for-case	format	warning suggested fix	translate() folds case over the ASCII alphabet alone. Use lower-case() or upper-case(), which are shorter and Unicode-aware.
unabbreviated-axis	format	warning safe fix	Verbose axis specifiers child::, attribute::, parent::node(), or self::node() are used. Replace them with the node name alone, @, .., or . respectively.
undefined-variable	format	error recommended	A variable is referenced where no binding in scope declares it. Declare it, or correct the name to one in scope.
unused-function-template-parameter	format	warning	A parameter is declared but never referenced in the function or template body. Remove it or use it.
use-double-slash	format	warning	A pattern contains //, which matches at any depth and is broader than intended. Use a specific path.
use-node-set-extension	format	warning safe fix	The node-set() extension function is unnecessary in XSLT 2.0 and later. Query the variable directly without it.
using-namespace-axis	format	warning	The namespace:: axis is deprecated in XSLT 2.0 and later. Use in-scope-prefixes() and namespace-uri-for-prefix() instead.