diff --git a/mkdocs/config/config_options.py b/mkdocs/config/config_options.py
index aacff403..9b16e302 100644
--- a/mkdocs/config/config_options.py
+++ b/mkdocs/config/config_options.py
@@ -10,6 +10,7 @@ import traceback
 import types
 import warnings
 from collections import Counter, UserString
+from types import SimpleNamespace
 from typing import (
     Any,
     Callable,
@@ -993,6 +994,12 @@ class MarkdownExtensions(OptionallyRequired[List[str]]):
             raise ValidationError(f"Invalid config options for Markdown Extension '{ext}'.")
         self.configdata[ext] = cfg
 
+    def pre_validation(self, config, key_name):
+        # To appease validation in case it involves the `!relative` tag.
+        config._current_page = current_page = SimpleNamespace()  # type: ignore[attr-defined]
+        current_page.file = SimpleNamespace()
+        current_page.file.src_path = ''
+
     def run_validation(self, value: object) -> list[str]:
         self.configdata: dict[str, dict] = {}
         if not isinstance(value, (list, tuple, dict)):
@@ -1037,6 +1044,7 @@ class MarkdownExtensions(OptionallyRequired[List[str]]):
         return extensions
 
     def post_validation(self, config: Config, key_name: str):
+        config._current_page = None  # type: ignore[attr-defined]
         config[self.configkey] = self.configdata
 
 
diff --git a/mkdocs/tests/build_tests.py b/mkdocs/tests/build_tests.py
index 275aad37..4b7596bf 100644
--- a/mkdocs/tests/build_tests.py
+++ b/mkdocs/tests/build_tests.py
@@ -814,7 +814,7 @@ class _TestPreprocessor(markdown.preprocessors.Preprocessor):
 
 class _TestExtension(markdown.extensions.Extension):
     def __init__(self, base_path: str) -> None:
-        self.base_path = base_path
+        self.base_path = str(base_path)
 
     def extendMarkdown(self, md: markdown.Markdown) -> None:
         md.preprocessors.register(_TestPreprocessor(self.base_path), "mkdocs_test", priority=32)
diff --git a/mkdocs/utils/yaml.py b/mkdocs/utils/yaml.py
index b0c0e7ad..46745eec 100644
--- a/mkdocs/utils/yaml.py
+++ b/mkdocs/utils/yaml.py
@@ -97,12 +97,13 @@ class RelativeDirPlaceholder(_DirPlaceholder):
         super().__init__(config, suffix)
 
     def value(self) -> str:
-        if self.config._current_page is None:
+        current_page = self.config._current_page
+        if current_page is None:
             raise exceptions.ConfigurationError(
                 "The current file is not set for the '!relative' tag. "
                 "It cannot be used in this context; the intended usage is within `markdown_extensions`."
             )
-        return os.path.dirname(self.config._current_page.file.abs_src_path)
+        return os.path.dirname(os.path.join(self.config.docs_dir, current_page.file.src_path))
 
 
 def get_yaml_loader(loader=yaml.Loader, config: MkDocsConfig | None = None):